x86: Code cleanup in strchr-evex and comment justifying branch

Small code cleanup for size: -81 bytes.

Add comment justifying using a branch to do NULL/non-null return.

All string/memory tests pass and no regressions in benchtests.

geometric_mean(N=20) of all benchmarks New / Original: .985
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

(cherry picked from commit ec285ea9)