[GlobalISel][NFC] Fix LLT Propagation (#119587)
Retain LLT type information by creating new LLTs from the original LLT
instead of only using the original scalar size.
This PR prepares for the [LLT FPInfo
RFC](https://discourse.llvm.org/t/rfc-globalisel-adding-fp-type-information-to-llt/83349/24)
where LLTs will carry additional floating point type information in
addition to the scalar size.
