[InstCombine] Skip scalable vectors in combineLoadToOperationType

Don't try to canonicalize loads to scalable vector types to loads
of integers.

This removes one assertion when trying to use a TypeSize as a parameter
to DataLayout::isLegalInteger. It does not handle the second part of the
function (which looks at bitcasts).

This patch also contains a NFC fix for Load Analysis, where a variable
initialization that would cause the same assertion is moved closer to
its use. This allows us to run the new test for InstCombine without
having to teach LocationSize to play nicely with scalable vectors.

Differential Revision: https://reviews.llvm.org/D70075