[X86] Add custom type legalization for v16i64->v16i8 truncate and v8i64->v8i8 truncat...

[X86] Add custom type legalization for v16i64->v16i8 truncate and v8i64->v8i8 truncate when v8i64 isn't legal

Summary:
The default legalization for v16i64->v16i8 tries to create a multiple stage truncate concatenating after each stage and truncating again. But avx512 implements truncates with multiple uops. So it should be better to truncate all the way to the desired element size and then concatenate the pieces using unpckl instructions. This minimizes the number of 2 uop truncates. The unpcks are all single uop instructions.

I tried to handle this by just custom splitting the v16i64->v16i8 shuffle. And hoped that the DAG combiner would leave the two halves in the state needed to make D68374 do the job for each half. This worked for the first half, but the second half got messed up. So I've implemented custom handling for v8i64->v8i8 when v8i64 needs to be split to produce the VTRUNCs directly.

Reviewers: RKSimon, spatel

Reviewed By: RKSimon

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68428

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@373864 91177308-0d34-0410-b5e6-96231b3b80d8