Optimization for the following SIGN_EXTEND pairs:
v8i8  -> v8i64, 
v8i8  -> v8i32, 
v4i8  -> v4i64, 
v4i16 -> v4i64 
for AVX and AVX2.

Bug 14865.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172708 91177308-0d34-0410-b5e6-96231b3b80d8