[x86][slh][NFC] Rm redundant liveness check

Patch by Zola Bridges!

From the review:

"""
In this changeset (https://reviews.llvm.org/D70283), I added a liveness
check everywhere the isDataInvariant* functions were used, so that I
could safely delete the checks within the function. I mistakenly left
that deletion out of the patch. The result is that the same condition is
checked twice for some instructions which is functionally fine, but not
good. This change deletes the redundant check that I intended to delete
in the last change.

This is the second of three patches that will make the data invariance
checks available for non-SLH passes and enable the FIXMEs related to
moving this metadata to the instruction tables to be resolved.

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

Differential Revision: https://reviews.llvm.org/D75650