AMDGPU: Strengthen some bfloat tests

Fix bitcast test, which was splitting apart phis intended to force
bitcasts that survive all the way to selection.

Disable the amdgpu-codegenprepare phi splitting, which defeats the technique
of using a phi to ensure a bitcast reaches all the way to selection. Also
add a variety of bfloat tests. These probably need revisiting to avoid the
cast folding into argument loads. Also round out set of bfloat bitcast and
ABI tests.

Add codegen tests for more bf16 operations The promotion of these works
contrary to the comment.