[InstCombine][X86] simplifyX86immShift - convert variable in-range vector...

[InstCombine][X86] simplifyX86immShift - convert variable in-range vector shift by immediate amounts to generic shifts (PR40391)

The slli/srli/srai 'immediate' vector shifts (although its not immediate anymore to match gcc) can be replaced with generic shifts if the shift amount is known to be in range.