Reapply [InstCombine] Don't check for alloc fn before fetching alloc size

Reapply the patch with getObjectSize() replaced by getAllocSize().
The former will also look through calls that return their argument,
and we'll end up placing dereferenceable attributes on intrinsics
like llvm.launder.invariant.group. While this isn't wrong, it also
doesn't seem to be particularly useful. For now, use getAllocSize()
instead, which sticks closer to the original behavior of this code.

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This code is just interested in the allocsize, not any other
allocator properties.