[llvm] [MIRLexer][RISCV] Eat a space after the Machine comment (PR #115365)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-aarch64-darwin` running on `doug-worker-5` while building `llvm` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/190/builds/9269

<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/MIR/skip-mir-comment-trailing-whitespace.mir' FAILED ********************
Exit Code: 2

Command Output (stderr):
--
RUN: at line 1: /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/MIR/skip-mir-comment-trailing-whitespace.mir -mtriple=riscv64 -mattr=+v -run-pass=none -o - | /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/MIR/skip-mir-comment-trailing-whitespace.mir
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/MIR/skip-mir-comment-trailing-whitespace.mir -mtriple=riscv64 -mattr=+v -run-pass=none -o -
+ /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/MIR/skip-mir-comment-trailing-whitespace.mir
/Users/buildbot/buildbot-root/aarch64-darwin/build/bin/llc: error: unable to get target for 'riscv64', see --version and --triple.FileCheck error: '<stdin>' is empty.
FileCheck command line:  /Users/buildbot/buildbot-root/aarch64-darwin/build/bin/FileCheck /Users/buildbot/buildbot-root/aarch64-darwin/llvm-project/llvm/test/CodeGen/MIR/skip-mir-comment-trailing-whitespace.mir

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/115365