nvc0/ir: add support for emitting partial min/max ops for int64

nvc0/ir: add support for emitting partial min/max ops for int64

These operations allow you to compute min/max on arbitrary-width
integers, 32 bits at a time.

Note that the low/med ops implicitly set the condition code, and the
med/high ops implicitly consume it.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>