[llvm] [IR] Initial introduction of memset_pattern (PR #97583)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `ml-opt-devrel-x86-64` running on `ml-opt-devrel-x86-64-b1` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/175/builds/8577

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/RISCV/memset-pattern.ll' FAILED ********************
Exit Code: 1

Command Output (stderr):
--
RUN: at line 2: /b/ml-opt-devrel-x86-64-b1/build/bin/llc < /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll -mtriple=riscv32 -mattr=+m    | /b/ml-opt-devrel-x86-64-b1/build/bin/FileCheck /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll --check-prefixes=RV32-BOTH,RV32
+ /b/ml-opt-devrel-x86-64-b1/build/bin/FileCheck /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll --check-prefixes=RV32-BOTH,RV32
+ /b/ml-opt-devrel-x86-64-b1/build/bin/llc -mtriple=riscv32 -mattr=+m
/b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll:17:19: error: RV32-BOTH-NEXT: expected string not found in input
; RV32-BOTH-NEXT: lw a2, 0(a1)
                  ^
<stdin>:9:36: note: scanning from here
# %bb.0: # %loadstoreloop.preheader
                                   ^
<stdin>:11:2: note: possible intended match here
 lw a3, 0(a1)
 ^
/b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll:85:14: error: RV32-NEXT: is not on the line after the previous match
; RV32-NEXT: add s4, a0, s4
             ^
<stdin>:68:2: note: 'next' match was here
 add s4, a0, s4
 ^
<stdin>:66:16: note: previous match ended here
 slli s4, a2, 4
               ^
<stdin>:67:1: note: non-matching line after previous match is here
 addi a2, a2, 1
^
/b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll:207:19: error: RV32-BOTH-NEXT: expected string not found in input
; RV32-BOTH-NEXT: lw a2, 0(a1)
                  ^
<stdin>:105:36: note: scanning from here
# %bb.0: # %loadstoreloop.preheader
                                   ^
<stdin>:107:2: note: possible intended match here
 lw a3, 0(a1)
 ^
/b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll:252:19: error: RV32-BOTH-NEXT: expected string not found in input
; RV32-BOTH-NEXT: lw a4, 0(a1)
                  ^
<stdin>:139:36: note: scanning from here
# %bb.1: # %loadstoreloop.preheader
                                   ^
<stdin>:141:2: note: possible intended match here
 lw a5, 0(a1)
 ^

Input file: <stdin>
Check file: /b/ml-opt-devrel-x86-64-b1/llvm-project/llvm/test/CodeGen/RISCV/memset-pattern.ll
...

```

</details>

https://github.com/llvm/llvm-project/pull/97583