GlobalISel: Lower scalarizing unmerge of a vector to shifts

AMDGPU sometimes has legal s16 and <2 x s16> operations, but all
registers are really 32-bit. An unmerge destination really should ben
widened to a 32-bit register. If widening a scalarizing vector with a
target size that matches the vector size, bitcast to integer and
extract the relevant bits with shifts.

I'm not sure if this is the right place for this. This could arguably
be part of widenScalar for the result. I also have a growing feeling
that we're missing a bitcast legalize action.

llvm-svn: 367604