Fix the handling of partial redefines in the fast register allocator.

Fix the handling of partial redefines in the fast register allocator.

A partial redefine needs to be treated like a tied operand, and the register
must be reloaded while processing use operands.

This fixes a bug where partially redefined registers were processed as normal
defs with a reload added. The reload could clobber another use operand if it was
a kill that allowed register reuse.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107193 91177308-0d34-0410-b5e6-96231b3b80d8