[ARM, inline-asm] Fix ARMTargetLowering::getRegForInlineAsmConstraint to return

register class tGPRRegClass if the target is thumb1.

This commit fixes a crash that occurs during register allocation which was
triggered when a virtual register defined by an inline-asm instruction had to
be spilled.
 
rdar://problem/18740489

llvm-svn: 221178