Fix bug in RegScavenger::scavengeRegister().

Fix bug in RegScavenger::scavengeRegister().

Reserved registers are not candidates for scavenging, and they were removed
from the candidate list like this:

CreateRegClassMask(RC, Candidates);
Candidates ^= ReservedRegs;

However, when there are reserved registers outside RC, this causes invalid
bits to be set in Candidates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75847 91177308-0d34-0410-b5e6-96231b3b80d8