[InstCombine] narrowFunnelShift - fold trunc/zext...

[InstCombine] narrowFunnelShift - fold trunc/zext or(shl(a,x),lshr(b,sub(bw,x))) -> fshl(a,b,x) (PR35155)

As discussed on PR35155, this extends narrowFunnelShift (recently renamed from narrowRotate) to support basic funnel shift patterns.

Unlike matchFunnelShift we don't include the computeKnownBits limitation as extracting the pattern from the zext/trunc layers should be a indicator of reasonable funnel shift codegen, in D89139 we demonstrated how to efficiently promote funnel shifts to wider types.

Differential Revision: https://reviews.llvm.org/D89542