i965/vec4: Fix liveness analysis with BRW_OPCODE_SEL

i965/vec4: Fix liveness analysis with BRW_OPCODE_SEL

We only consider a vgrf defined by a given block if the block writes to it
unconditionally. So far we have been checking this by testing that the
instruction is not predicated, however, in the case of BRW_OPCODE_SEL,
the predication is used to select the value to write, not to decide if
the write is actually done. The consequence of this was increased life
spans for affected vgrfs, which could lead to additional register pressure.

Since NIR generates selects for conditional writes this was causing massive
register pressure in a handful of piglit and dEQP tests that had a large
number of select operations with the NIR-vec4 backend.

Fixes the following piglit tests with the NIR-vec4 backend:
spec/glsl-1.50/execution/variable-indexing/vs-output-array-vec4-index-wr-before-gs
spec/glsl-1.50/execution/variable-indexing/gs-input-array-vec4-index-rd
spec/glsl-1.50/execution/variable-indexing/vs-output-array-vec2-index-wr-before-gs
spec/glsl-1.50/execution/variable-indexing/vs-output-array-vec3-index-wr-before-gs
spec/glsl-1.50/execution/variable-indexing/vs-output-array-float-index-wr-before-gs

Fixes 80 dEQP tests with the NIR-vec4 backend in the following category:
dEQP-GLES3.functional.ubo.*

Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>