Handle regmasks in Thumb1RegisterInfo::saveScavengerRegister().

This function could have r12 live across a function call when compiling
thumb1 code.

The test case for this is not included because it is very long. It must
provoke emergency spilling near a function call. The behavior is
provoked by MultiSource/Applications/JM/lencod, and it triggers an
assertion in the scavenger.

<rdar://problem/10963642>

llvm-svn: 151855