[InstCombine] Fix incorrect usage of getPrimitiveSizeInBits when we should be...

[InstCombine] Fix incorrect usage of getPrimitiveSizeInBits when we should be using the element size for vectors

For vectors, getPrimitiveSizeInBits returns the full vector width. This code should using the element size for vectors. This could be fixed by calling getScalarSizeInBits, but its even easier to just get it from the APInt we're checking.

Differential Revision: https://reviews.llvm.org/D51938

llvm-svn: 341971