[AMDGPU] Promote <2 x bfloat> select to i32 (#214598)
`select` on <2 x bfloat> has no operation action registered, so it
defaults to Expand. The vector legalizer scalarizes it into two 16-bit
selects, which are promoted to i16 and selected as a pair of
V_CNDMASK_B16 plus shifts and a repack to reassemble the result on
targets without true16.
