ac/llvm: open code fpow on LLVM 12 using fmul.legacy

ac/llvm: open code fpow on LLVM 12 using fmul.legacy

A quick look at the asm shows that this enables source modifiers
(neg, abs) for v_mul_legacy_f32.

Totals from affected shaders:
SGPRS: 110104 -> 110400 (0.27 %)
VGPRS: 57632 -> 57636 (0.01 %)
Spilled SGPRs: 66 -> 63 (-4.55 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 3290412 -> 3283068 (-0.22 %) bytes
Max Waves: 32141 -> 32141 (0.00 %)

Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9395>