RBIT Instruction only available for ARMv6t2 and above.

Summary:
r255334 matches bit-reverse pattern in InstCombine and generates calls to Instrinsic::bitreverse.

RBIT instruction is only available for ARMv6t2 and above. This patch has the intrinsic expanded during legalization for ARMv4 and ARMv5.

Patch by Z. Zheng <zhaoshiz@codeaurora.org>

Reviewers: apazos, jmolloy, weimingz

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: http://reviews.llvm.org/D15932

llvm-svn: 257188