[X86] Improve i8 + 'slow' i16 funnel shift codegen

This is a preliminary patch before I deal with the xor+and issue raised in D77301.

We get much better code for i8/i16 funnel shifts by concatenating the operands together and performing the shift as a double width type, it avoids repeated use of the shift amount and partial registers.

fshl(x,y,z) -> (((zext(x) << bw) | zext(y)) << (z & (bw-1))) >> bw.
fshr(x,y,z) -> (((zext(x) << bw) | zext(y)) >> (z & (bw-1))) >> bw.

Alive2: http://volta.cs.utah.edu:8080/z/CZx7Cn

This doesn't do as well for i32 cases on x86_64 (the xor+and followup patch is much better) so I haven't bothered with that.

Cases with constant amounts are more dubious as well so I haven't currently bothered with those - its these kind of 'edge' cases that put me off trying to put this in TargetLowering::expandFunnelShift.

Differential Revision: https://reviews.llvm.org/D80466