[llvm] [RISCV] Rework memcpy test (PR #120364)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `premerge-monolithic-windows` running on `premerge-windows-1` while building `llvm` at step 8 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/35/builds/6086

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

```
Step 8 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: CodeGen/RISCV/memcpy.ll' FAILED ********************
Exit Code: 1

Command Output (stdout):
--
# RUN: at line 2
c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe < C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll -mtriple=riscv32    | c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll --check-prefixes=RV32-BOTH,RV32
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\llc.exe' -mtriple=riscv32
# executed command: 'c:\ws\buildbot\premerge-monolithic-windows\build\bin\filecheck.exe' 'C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll' --check-prefixes=RV32-BOTH,RV32
# .---command stderr------------
# | C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll:278:14: error: RV32-NEXT: expected string not found in input
# | ; RV32-NEXT: lbu a2, 14(a1)
# |              ^
# | <stdin>:122:18: note: scanning from here
# | # %bb.0: # %entry
# |                  ^
# | <stdin>:123:2: note: possible intended match here
# |  li a2, 15
# |  ^
# | C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll:371:14: error: RV32-NEXT: expected string not found in input
# | ; RV32-NEXT: lbu a2, 15(a1)
# |              ^
# | <stdin>:132:18: note: scanning from here
# | # %bb.0: # %entry
# |                  ^
# | <stdin>:133:2: note: possible intended match here
# |  li a2, 16
# |  ^
# | C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll:468:14: error: RV32-NEXT: expected string not found in input
# | ; RV32-NEXT: lbu a2, 30(a1)
# |              ^
# | <stdin>:142:18: note: scanning from here
# | # %bb.0: # %entry
# |                  ^
# | <stdin>:143:2: note: possible intended match here
# |  li a2, 31
# |  ^
# | C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll:861:14: error: RV32-NEXT: expected string not found in input
# | ; RV32-NEXT: lbu a2, 30(a1)
# |              ^
# | <stdin>:271:18: note: scanning from here
# | # %bb.0: # %entry
# |                  ^
# | <stdin>:272:2: note: possible intended match here
# |  li a2, 31
# |  ^
# | 
# | Input file: <stdin>
# | Check file: C:\ws\buildbot\premerge-monolithic-windows\llvm-project\llvm\test\CodeGen\RISCV\memcpy.ll
# | 
...

```

</details>

https://github.com/llvm/llvm-project/pull/120364