[AMDGPU] Enable scheduling around FP MODE-setting instructions

Pre-gfx10 all MODE-setting instructions were S_SETREG_B32 which is
marked as having unmodeled side effects, which makes the machine
scheduler treat it as a barrier. Now that we have proper implicit $mode
operands we can use a no-side-effects S_SETREG_B32_mode pseudo instead
for setregs that only touch the FP MODE bits, to give the scheduler more
freedom.

Differential Revision: https://reviews.llvm.org/D87446