tgsi: rename the TGSI fragment kill opcodes

tgsi: rename the TGSI fragment kill opcodes

TGSI_OPCODE_KIL and KILP had confusing names.  The former was conditional
kill (if any src component < 0).  The later was unconditional kill.
At one time KILP was supposed to work with NV-style condition
codes/predicates but we never had that in TGSI.

This patch renames both opcodes:
  TGSI_OPCODE_KIL -> KILL_IF   (kill if src.xyzw < 0)
  TGSI_OPCODE_KILP -> KILL     (unconditional kill)

Note: I didn't just transpose the opcode names to help ensure that I
didn't miss updating any code anywhere.

I believe I've updated all the relevant code and comments but I'm
not 100% sure that some drivers had this right in the first place.
For example, the radeon driver might have llvm.AMDGPU.kill and
llvm.AMDGPU.kilp mixed up.  Driver authors should review their code.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>