nv50/ir: pre-compute BFE arg when both bits and offset are imm

nv50/ir: pre-compute BFE arg when both bits and offset are imm

Due to a quirk in how the nv50 opt passes run, the algebraic
optimization that looks for these BFE's happens before the constant
folding pass. Rearranging these passes isn't a great idea, but this is
easy enough to fix. Allows a following cvt to eliminate the bfe in
certain situations.

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