[RISCV] XTHeadMemPair: Fix invalid mempair combine for types other than i32/i64

A mistake in the control flow of performMemPairCombine resulted in paired
loads/stores for types that were not supported by the instructions (i8/i16).
These loads/stores could not match the constraints of the patterns defined
in the THead td file and the compiler would throw a 'Cannot select' error.

This is now fixed and two new test functions have been added in xtheadmempair.ll
which would previously crash the compiler. The compiler was additionally tested
with a wide range of benchmarks and no issues were observed.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D144559