nir/opcodes: Make ldexp take an explicitly 32-bit int

nir/opcodes: Make ldexp take an explicitly 32-bit int

There is no sense in having the double version of ldexp take a 64-bit
integer.  Instead, let's just take a 32-bit int all the time.  This also
matches what GLSL does where both variants of ldexp take a regular integer
for the exponent argument.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>