Reland [GlobalISel] NFC: Have LLT::getSizeInBits/Bytes return a TypeSize.

This patch relands https://reviews.llvm.org/D104454, but fixes some failing
builds on Mac OS which apparently has a different definition for size_t,
that caused 'ambiguous operator overload' for the implicit conversion
of TypeSize to a scalar value.

This reverts commit b732e6c9.