[NFC] Use ArgListEntry indirect types more in ISel lowering

For opaque pointers, we're trying to avoid uses of
PointerType::getElementType().

A couple of ISel places use PointerType::getElementType(). Some of these
are easy to fix by using ArgListEntry's indirect types.

The inalloca type wasn't stored there, as opposed to preallocated and
byval which have their indirect types available, so add it and use it.

Differential Revision: https://reviews.llvm.org/D101713