[X86] In lowerVectorShuffleAsElementInsertion, if were able to find a scalar...

[X86] In lowerVectorShuffleAsElementInsertion, if were able to find a scalar i8 or i16 and need to zero extend it, make sure we use a vXi32 type of the full vector width.

Previously, this was hardcoded to v4i32, but if the input type is 256 bits we need to use v8i32.

Fixes PR35443

llvm-svn: 319208