[GlobalISel] Use GCDTy when extracting GCD ty from leftover regs in insertParts

`LegalizerHelper::insertParts` uses `extractGCDType` on registers split into
a desired type and a smaller leftover type. This is used to populate a list
of registers. Each register in the list will have the same type as returned by
`extractGCDType`.

If we have

- `ResultTy` = s792
- `PartTy` = s64
- `LeftoverTy` = s24

When we call `extractGCDType`, we'll end up with two different types appended
to the list:

Part: gcd(792, 64, 24) => s8
Leftover: gcd(792, 24, 24) => s24

When this happens, we'll hit an assert while trying to build a G_MERGE_VALUES.

This patch changes the code for the leftover type so that we reuse the GCD from
the desired type.

e.g.

Leftover: gcd(792, 8, 24) => s8

https://llvm.godbolt.org/z/137Kqxj6j

Differential Revision: https://reviews.llvm.org/D105674