[MC][x86-64] Decide if each individual mergeable constants is small or large (#209881)
Rather than only relying on the code model. Otherwise we can end up with
the IR and MC disagreeing about which ones are small/large.
Don't use MergeableConst*Section in
TargetLoweringObjectFileELF::getSectionForConstant(), decide which
section dynamically. Other binary formats still use those though.
This resolves some relocation overflows from mixed small/medium code
model code.
Followup to #190903.
Assisted-by: Gemini