This saves materializing the immediate. The additional forms are less
common (they don't usually show up for bitfield insert/extract), but
they're still relevant.
I had to add a new target hook to prevent DAGCombine from reversing the
transform. That isn't the only possible way to solve the conflict, but
it seems straightforward enough.
Differential Revision: https://reviews.llvm.org/D55630
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349857 91177308-0d34-0410-b5e6-96231b3b80d8
Eli Friedman
8e797de70d