[IR] Reduce max supported integer from 2^24-1 to 2^23.

SelectionDAG will promote illegal types up to a power of 2 before
splitting down to a legal type. This will create an IntegerType
with a bit width that must be <= MAX_INT_BITS. This places an
effective upper limit on any type of 2^23 so that we don't try
create a 2^24 type.

I considered putting a fatal error somewhere in the path from
TargetLowering::getTypeConversion down to IntegerType::get, but
limiting the type in IR seemed better.

This breaks backwards compatibility with IR that is using a really
large type. I suspect such IR is going to be very rare due to the
the compile time costs such a type likely incurs.

Prevents the ICE in PR51829.

Reviewed By: efriedma, aaron.ballman

Differential Revision: https://reviews.llvm.org/D109721