llvm-tblgen: Mangle operand replacements into the strings in printAliasInstr.

Cuts down the bloat in the AArch64 asm writer a bit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190527 91177308-0d34-0410-b5e6-96231b3b80d8