[AArch64] Take advantage of variable shift/rotate amount implicit mod operation.

Summary:
Optimize code generated for variable shifts/rotates by taking advantage
of the implicit and/mod done on the variable shift amount register.

Resolves bug 27582 and bug 37421.

Reviewers: t.p.northover, qcolombet, MatzeB, javed.absar

Subscribers: rengolin, kristof.beyls, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D46844

llvm-svn: 333214