nir: Add a no wrapping bits to nir_alu_instr

nir: Add a no wrapping bits to nir_alu_instr

They indicate the operation does not cause overflow or underflow.
This is motivated by SPIR-V decorations NoSignedWrap and
NoUnsignedWrap.

Change the storage of `exact` to be a single bit, so they pack
together.

v2: Handle no_wrap in nir_instr_set.  (Karol)

v3: Use two separate flags, since the NIR SSA values and certain
    instructions are typeless, so just no_wrap would be insufficient
    to know which one was referred to.  (Connor)

v4: Don't use nir_instr_set to propagate the flags, unlike `exact`,
    consider the instructions different if the flags have different
    values.  Fix hashing/comparing.  (Jason)

Reviewed-by: Karol Herbst <kherbst@redhat.com> [v1]
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>