[X86] Make the MUL->VPMADDWD work before op legalization on AVX1 targets....

[X86] Make the MUL->VPMADDWD work before op legalization on AVX1 targets. Simplify feature checks by using isTypeLegal.

The v8i32 conversion on AVX1 targets was only working after LowerMUL splits 256-bit vectors.

While I was there I've also made it so we don't have to check for AVX2 and BWI directly and instead just ask if the type is legal.

Differential Revision: https://reviews.llvm.org/D44190

llvm-svn: 326917