[llvm] Enable logf128 constant folding for hosts with 128bit long double (PR #96287)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ppc64le-lld-multistage-test` running on `ppc64le-lld-multistage-test` while building `llvm` at step 7 "test-build-stage1-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/168/builds/1916

Here is the relevant piece of the build log for the reference:
```
Step 7 (test-build-stage1-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: Transforms/InstSimplify/ConstProp/logf128.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 2: /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage1/bin/opt < /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll -passes=instsimplify -S | /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage1/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage1/bin/opt -passes=instsimplify -S
+ /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/build/stage1/bin/FileCheck /home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:10:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xL300000000000000040010A2B23F3BAB7
              ^
<stdin>:9:27: note: scanning from here
define fp128 @log_e_64() {
                          ^
<stdin>:10:43: note: possible intended match here
 %A = call fp128 @llvm.log.f128(fp128 noundef 0xL00000000000000004005000000000000)
                                          ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:18:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xL3000000000000000C00C654628220780
              ^
<stdin>:14:59: note: scanning from here
define fp128 @log_e_smallest_positive_subnormal_number() {
                                                          ^
<stdin>:15:43: note: possible intended match here
 %A = call fp128 @llvm.log.f128(fp128 noundef 0xL00000000000000010000000000000000)
                                          ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:26:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xLD000000000000000C00C62D918CE2421
              ^
<stdin>:19:49: note: scanning from here
define fp128 @log_e_largest_subnormal_number() {
                                                ^
<stdin>:21:2: note: possible intended match here
 ret fp128 %A
 ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:35:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xLD000000000000000C00C62D918CE2421
              ^
<stdin>:24:56: note: scanning from here
define fp128 @log_e_smallest_positive_normal_number() {
                                                       ^
<stdin>:25:43: note: possible intended match here
 %A = call fp128 @llvm.log.f128(fp128 noundef 0xL00000000000000000001000000000000)
                                          ^
/home/buildbots/llvm-external-buildbots/workers/ppc64le-lld-multistage-test/ppc64le-lld-multistage-test/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:43:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xLF000000000000000400C62E42FEFA39E
              ^
<stdin>:29:46: note: scanning from here
define fp128 @log_e_largest_normal_number() {
                                             ^
...

```

https://github.com/llvm/llvm-project/pull/96287