[X86] Pass v32i16/v64i8 in zmm registers on KNL target.

[X86] Pass v32i16/v64i8 in zmm registers on KNL target.

gcc and icc pass these types in zmm registers in zmm registers.

This patch implements a quick hack to override the register
type before calling convention handling to one that is legal.
Longer term we might want to do something similar to 256-bit
integer registers on AVX1 where we just split all the operations.

Fixes PR42957

Differential Revision: https://reviews.llvm.org/D66708

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@370495 91177308-0d34-0410-b5e6-96231b3b80d8