glsl: Fix constant expression handling for <, >, <=, >= on vectors.

glsl: Fix constant expression handling for <, >, <=, >= on vectors.

ir_binop_less, ir_binop_greater, ir_binop_lequal, and ir_binop_gequal
are defined to work on vectors as well as scalars, as long as the two
operands have the same type.

This is evident from both ir_validate.cpp and our use of these opcodes
in the GLSL lessThan, greaterThan, lessThanEqual, greaterThanEqual
built-in functions.

Found by code inspection.  Not known to fix any bugs.  Presumably, our
tests for the built-in comparison functions must pass because C.E.
handling is done on the ir_call of "greaterThan" rather than the inlined
opcode.  The C.E. handling of the built-in function calls is correct.

NOTE: This is a candidate for the 7.9 branch.