freedreno/ir3: Enable core NIR's 16-bit ALU optimizations.
In addition to hopefully generating shorter code, this optimizes out a comparison of a mediump-cast value in dEQP-GLES2.functional.shaders.algorithm.rgb_to_hsl_fragment passed through ANGLE, and allows the test to pass. We believe it to be a test bug, but emitting better code like apparently everyone else does is also a fine result.
No change on GLES gfxbench shaders.
Fixes: #6585 (closed)