[X86][NFCI] Remove target-specific branch optimisation that's handled in BranchFolding

This specific optimisation is handled in OptimizeBlock in BranchFolding
so is redundant. As discussed on the review thread, I've verified that
we have test coverage for that optimisation within test/CodeGen/X86 by
disabling the BranchFolding version of this transform after applying
this patch and rerunning the test suite.

Differential Revision: https://reviews.llvm.org/D129204