Do not extend extension results beyond the use of a PHI instruction at the start of a use block. A PHI use is expected to kill its source values.

llvm-svn: 93895

Commit some changes I had managed to lose last night while refactoring the code. Avoid change use of PHI instructions because it's not legal to insert any instructions before them.
This fixes PR6027.

llvm-svn: 93335

llvm-svn: 93306

Add a quick pass to optimize sign / zero extension instructions. For targets where the pre-extension values are available in the subreg of the result of the extension, replace the uses of the pre-extension value with the result + extract_subreg.

For now, this pass is fairly conservative. It only perform the replacement when both the pre- and post- extension values are used in the block. It will miss cases where the post-extension values are live, but not used.

llvm-svn: 93278