[SystemZ] Bugfix in expandRxSBG()

Csmith discovered a program that caused wrong code generation with -O0:

When handling a SIGN_EXTEND in expandRxSBG(), RxSBG.BitSize may be less than
the Input width (if a truncate was previously traversed), so maskMatters()
should be called with a masked based on the width of the sign extend result
instead.

Review: Ulrich Weigand
llvm-svn: 319892