[LowerTypeTests] Avoid creation of select constant expression

[LowerTypeTests] Avoid creation of select constant expression

LowerTypeTests replaces weak declarations with an icmp+select
constant expressions. As this is not a relocatable expression,
it additionally promotes initializers using it to global ctors.

As part of https://discourse.llvm.org/t/rfc-remove-most-constant-expressions/63179,
I would like to remove the select constant expression, of which LTT
is now the last user. This is a bit tricky, because we now need to
replace a constant with an instruction, which might require
converting intermediate constant expression users to instructions as
well.

We do this using the convertUsersOfConstantsToInstructions() helper.
However, it needs to be slightly extended to also support expansion
of ConstantAggregates. These are important in this context, because
the promotion of initializers to global ctors will produce stores
of such aggregates.

Differential Revision: https://reviews.llvm.org/D145247