[llvm] [X86][GISel] Add initial tests for x86 postleg combiner (PR #183844)

[Matt Arsenault via llvm-commits]

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+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple x86_64 -run-pass=X86-postlegalizer-combiner -verify-machineinstrs %s -o - | FileCheck %s
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arsenm wrote:

```suggestion
# RUN: llc -mtriple x86_64 -run-pass=X86-postlegalizer-combiner %s -o - | FileCheck %s
```

https://github.com/llvm/llvm-project/pull/183844