spirv: fix OpBitcast when the src and dst bitsize are different (v3)

spirv: fix OpBitcast when the src and dst bitsize are different (v3)

Before, we were just implementing it with a move, which is incorrect
when the source and destination have different bitsizes. To implement
it properly, we need to use the 64-bit pack/unpack opcodes. Since
glslang uses OpBitcast to implement packInt2x32 and unpackInt2x32, this
should fix them on anv (and radv once we enable the int64 capability).

v2: make supporting non-32/64 bit easier (Jason)
v3: add another assert (Jason)

Fixes: b3135c3c ("anv: Advertise shaderInt64 on Broadwell and above")
Signed-off-by: Connor Abbott <cwabbott0@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>