[InstCombine][SDAG] Add range attributes to shamts on fshl/fshr in `matchFunnelShift`

[dtcxzyw]

In https://reviews.llvm.org/D88783, we fold or(shl(a,x),lshr(b,sub(bw,x))) -> fshl(a,b,x) iff x < bw. This constraint on shamts is added to avoid redundant and x, 2^bw - 1 in the codegen. However, this may increase the complexity of ISel and produce inefficient code. See an example from https://github.com/cyyself/hwprobe/blob/master/ifunc_example/ror32.c: https://godbolt.org/z/fxEb1f65h.

To address this issue, we can add attribute range(iN 0, N) to the shamt in InstCombine. Then we use assertzext in SelectionDAGBuilder to eliminate unnecessary and insts.

Looking forward to your inputs.
cc @nikic @topperc @RKSimon @asb @preames @cyyself

[nikic]

Sounds reasonable to me.