[SystemZ] Enable index register memory constraints for inline ASM

This enables use of the 'R' and 'T' memory constraints for inline ASM
operands on SystemZ, which allow an index register as well as an
immediate displacement. This patch includes corresponding documentation
and test case updates.

As with the last patch of this kind, I moved the 'm' constraint to the
most general case, which is now 'T' (base + 20-bit signed displacement +
index register).

Author: colpell
Differential Revision: http://reviews.llvm.org/D21239

llvm-svn: 272547