AMDGPU/GlobalISel: Mostly fix BFI patterns

Most importantly, fixes constant bus errors in the 64-bit cases. It's
surprising to me these were even passing the selection test using
SReg_* sources. Also fixes pattern matching in the 32-bit cases, with
simple operands.

These patterns aren't working in a few cases, like with mixed SGPR
inputs. The patterns aren't looking through the SGPR->VGPR copies like
they need to. The vector cases also have some unmerges of build_vector
which are obscuring the inputs.