[RISCV] Check type is legal before combining mgather to vlse intrinsic (#81107)

Otherwise we will crash since target intrinsics don't have their types
legalized. Let the mgather get legalized first, then do the combine on
the legal type.
Fixes #81088

Co-authored-by: Craig Topper <craig.topper@sifive.com>
(cherry picked from commit 06c89bd5)