Reland "VectorUtils: mark xrint as trivially vectorizable" (#71416)

With the recent change 98c90a13 (ISel: introduce vector ISD::LRINT,
ISD::LLRINT; custom RISCV lowering), it is now possible for
SLPVectorizer, LoopVectorize, and Scalarizer to operate on llvm.lrint
and llvm.llrint, with vector codegen for the RISC-V target. Make a
trivial change to VectorUtils, and update the corresponding tests.

A couple of important fixes have been landed since the original patch
was landed and reverted, and it is now safe to re-land the patch:
5e1d81ac (LegalizeIntegerTypes: implement PromoteIntRes for xrint) and
fd887a36 (LegalizeVectorTypes: fix bug in widening of vec result in
xrint). See also #71399, which proves that lrint and llrint will indeed
produce vector codegen on RISC-V.

Fixes #55208.