[SystemZ] Support LRVH and STRVH opcodes

Summary: On Linux, /usr/include/bits/byteswap-16.h defines __byteswap_16(x) as an inlined LRVH (Load Reversed Half-word) instruction. The SystemZ back-end did not support this opcode and the inlined assembly would cause a fatal error.

Reviewers: bryanpkc, uweigand

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18732

llvm-svn: 269688