SimplifyLibCalls: Don't require ldexp to emit intrinsic in exp2 combine (#92707)

When folding exp2(itofp(x)) to ldexp(1, x), don't require an ldexp
libcall to emit the intrinsic.

The intrinsic needs to be handled regardless of whether the system has a
libcall, and we have an inline implementation of ldexp already. This
fixes the instance in the exp2->ldexp fold. Another instance exists for
the pow(2) -> ldexp case

The LTO test change isn't ideal, since it's just moving the problem to
another instance where we're relying on implied libm behavior for an
intrinsic transform. Use exp10 since that's a harder case to solve in
the libcall house of cards we have.