[InstCombine][X86] Teach how to fold calls to SSE2/AVX2 packed logical shift
right intrinsics.

A packed logical shift right with a shift count bigger than or equal to the
element size always produces a zero vector. In all other cases, it can be
safely replaced by a 'lshr' instruction.
[LCG] Refactor the duplicated code I added in my last commit here into
a helper function. Also factor the other two places where we did the
same thing into the helper function. =] Much cleaner this way. NFC.