[RISCV] Fix crash when legalizing mgather/scatter on rv32

This is a fix for a subset of legalization problems around 64 bit indices on
rv32 targets.  For RV32+V, we were using the wrong mask type for the manual
truncation lowering for fixed length vectors.  Instead, just use the generic
TRUNCATE node, and let it be lowered as needed.

Note that legalization is still broken for rv32+zve32.  That appears to be
a different issue.