[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 `llvm-clang-x86_64-gcc-ubuntu` running on `sie-linux-worker3` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/174/builds/3029

Here is the relevant piece of the build log for the reference:
```
Step 6 (test-build-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/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/opt < /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll -passes=instsimplify -S | /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/FileCheck /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll
+ /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/build/bin/opt -passes=instsimplify -S
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:75:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xL00000000000000007FFF800000000000
              ^
<stdin>:41:35: note: scanning from here
define fp128 @log_e_negative_2() {
                                  ^
<stdin>:42:2: note: possible intended match here
 ret fp128 0xL0000000000000000FFFF800000000000
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:107:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret fp128 0xL00000000000000007FFF800000000000
              ^
<stdin>:57:42: note: scanning from here
define fp128 @log_e_negative_infinity() {
                                         ^
<stdin>:58:2: note: possible intended match here
 ret fp128 0xL0000000000000000FFFF800000000000
 ^
/home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll:123:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: ret <2 x fp128> <fp128 0xL00000000000000007FFF800000000000, fp128 0xL00000000000000007FFF800000000000>
              ^
<stdin>:65:48: note: scanning from here
define <2 x fp128> @log_e_negative_2_vector() {
                                               ^
<stdin>:66:2: note: possible intended match here
 ret <2 x fp128> <fp128 0xL0000000000000000FFFF800000000000, fp128 0xL0000000000000000FFFF800000000000>
 ^

Input file: <stdin>
Check file: /home/buildbot/buildbot-root/llvm-clang-x86_64-gcc-ubuntu/llvm-project/llvm/test/Transforms/InstSimplify/ConstProp/logf128.ll

-dump-input=help explains the following input dump.

Input was:
<<<<<<
             1: ; ModuleID = '<stdin>' 
             2: source_filename = "<stdin>" 
             3:  
             4: ; Function Attrs: nocallback nofree nosync nounwind speculatable willreturn memory(none) 
             5: declare fp128 @llvm.log.f128(fp128) #0 
             6:  
             7: declare fp128 @logl(fp128) 
...

```

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