This patch introduces a new ISel pattern for
v_bfi instructions. In some cases, only a single
v_bfi instruction is generated, even if there
could be multiple ones. The final codegen has
leftover v_and and v_xor instructions.
Such cases can appear when using nested bitfieldInsert
A (xor (and imm0, (xor (shl), (xor (and (xor (shl)), imm1)))) has two BFI parts.
The outer BFI part relies on the inner BFI part. During InstCombine, the inner xor
sequence gets turned into bfi_0 = (y & x) | (z & ~x) and later to a BFI, while the
outer BFI part stays untouched and will not be converted into a BFI instruction.