nir/int64: Properly handle imod/irem

nir/int64: Properly handle imod/irem

The previous implementation was fine for GLSL which doesn't really have
a signed modulus/remainder.  They just leave the behavior undefined
whenever either source is negative.  However, in SPIR-V, there is a
defined behavior for negative arguments.  This commit beefs up the pass
so that it handles both correctly.  Tested using a hacked up version of
the Vulkan CTS test to get 64-bit support.

Reviewed-by: Matt Turner <mattst88@gmail.com>