[LV] Collect a list of all element types found in the loop (NFC)

Splits `getSmallestAndWidestTypes` into two functions, one of which now collects
a list of all element types found in the loop (`ElementTypesInLoop`). This ensures we do not
have to iterate over all instructions in the loop again in other places, such as in D102253
which disables scalable vectorization of a loop if any of the instructions use invalid types.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D105437