[llvm] [SPIR-V] Fix dangling MachineFunction pointers in finalizeLowering (PR #196050)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Windows-->
# :window: Windows x64 Test Results

* 134345 tests passed
* 3231 tests skipped
* 1 test failed

## Failed Tests
(click on a test name to see its output)

### LLVM-Unit
<details>
<summary>LLVM-Unit.Target/SPIRV/_/SPIRVTests_exe/SPIRVGlobalRegistryTest/PrepareFunctionsClearsStalePointers</summary>

```
Script:
--
C:\_work\llvm-project\llvm-project\build\unittests\Target\SPIRV\.\SPIRVTests.exe --gtest_filter=SPIRVGlobalRegistryTest.PrepareFunctionsClearsStalePointers
--
C:\_work\llvm-project\llvm-project\llvm\unittests\Target\SPIRV\SPIRVGlobalRegistryTests.cpp:102
Expected equality of these values:
  GR->findDeducedElementType(F)
    Which is: 0000015D470A37D8
  nullptr
    Which is: (nullptr)


```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/196050