This ugly patch works around a GCC bug where it is compiling SelectCode to

use too much stack space, overflowing the stack for large functions.  Instead
of emitting new SDOperands in each match block, we emit some common ones at
the top of SelectCode then reuse them when possible.

This reduces the stack size of SelectCode from 28K to 21K.  Note that GCC
compiles it to 512 bytes :-/

I've filed GCC PR 25505 to track this.

llvm-svn: 24882