Cleanup x86_mmx after removing IR type  (#100646)

After #98505, the textual IR keyword `x86_mmx` was temporarily made to
parse as `<1 x i64>`, so as not to require a lot of test update noise.

This completes the removal of the type, by removing the`x86_mmx` keyword
from the IR parser, and making the (now no-op) test updates via `sed -i
's/\bx86_mmx\b/<1 x i64>/g' $(git grep -l x86_mmx llvm/test/)`.
Resulting bitcasts from <1 x i64> to itself were then manually deleted.

Changes to llvm/test/Bitcode/compatibility-$VERSION.ll were reverted, as
they're intended to be equivalent to the .bc file, if parsed by old
LLVM, so shouldn't be updated.

A few tests were removed, as they're no longer testing anything, in the
following files:
- llvm/test/Transforms/GlobalOpt/x86_mmx_load.ll
- llvm/test/Transforms/InstCombine/cast.ll
- llvm/test/Transforms/InstSimplify/ConstProp/gep-zeroinit-vector.ll

Works towards issue #98272.