[x86] use zero-extending load of a byte outside of loops too

This implements the main suggested change from issue #56498.
Using the shorter (non-extending) instruction with only
-Oz ("minsize") rather than -Os ("optsize") is left as a
possible follow-up.

As noted in the bug report, the zero-extending load may have
shorter latency/better throughput across a wide range of x86
micro-arches, and it avoids a potential false dependency.
The cost is an extra instruction byte.

This could cause perf ups and downs from secondary effects,
but I don't think it is possible to account for those in
advance, and that will likely also depend on exact micro-arch.
This does bring LLVM x86 codegen more in line with existing
gcc codegen, so if problems are exposed they are more likely
to occur for both compilers.

Differential Revision: https://reviews.llvm.org/D129775