Fix left shifts of negative integers in AArch64 InstPrinter/Disassembler

Summary:
Left shift of negative integer is an undefined behavior, and
is reported by UBSan. It's ok for imm values to be negative, so we can
just replace left shifts with multiplications.

Test Plan: check-llvm test suite

Reviewers: t.p.northover

Reviewed By: t.p.northover

Subscribers: aemerson, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D5132

llvm-svn: 216910