AArch64: correctly fast-isel i8 & i16 multiplies

We were asking for a register for type i8 or i16 which caused an assert.

rdar://problem/17620015

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212718 91177308-0d34-0410-b5e6-96231b3b80d8