[SDAG] Convert FSHL <--> FSHR if the target only supports one of them

D77152 tried to do this but got it wrong in the shift-by-zero case.
D86430 reverted the wrong code. Reimplement the optimization with
different code depending on whether the shift amount is known to be
non-zero (modulo bitwidth).

This improves code quality for fshl tests on AMDGPU, which only has an
fshr instruction.

Differential Revision: https://reviews.llvm.org/D86438