Adds rewrite patterns to convert select+cmp instructions into clamp
instructions whenever possible. Support is added to convert:
- FOrdLessThan, FOrdLessThanEqual to GLSLFClampOp.
- SLessThan, SLessThanEqual to GLSLSClampOp.
- ULessThan, ULessThanEqual to GLSLUClampOp.