[x86][slh] Move isDataInvariant* functions

Patch by Zola Bridges!

From the review:

"""
I moved these functions to X86InstrInfo.cpp, so they are available from
another pass. In addition, this is a step toward resolving the FIXME to
move this metadata to the instruction tables.

This is the final step to make these two data invariance checks
available for non-SLH passes.

The other two steps were here:

- https://reviews.llvm.org/D70283
- https://reviews.llvm.org/D75650

Tested via llvm-lit llvm/test/CodeGen/X86/speculative-load-hardening*
"""

Differential Revision: https://reviews.llvm.org/D75654