[X86] Correct the implementation of BEXTR load folding to use the shift as the...

[X86] Correct the implementation of BEXTR load folding to use the shift as the parent node and pass a separate root.

We were calling tryFoldLoad with the 'and' node was the root and parent node of the load. But the parent of the load should be the shift that proceeds the and. While the and node is correctly the root node.

To fix this I had to make tryFoldLoad take a separate use and root input. I've added a convenience version with the old signature to avoid updating the other call sites.

llvm-svn: 317720