[DAGCombiner][RISCV] Don't use vector types in...

[DAGCombiner][RISCV] Don't use vector types in DAGCombiner::tryStoreMergeOfLoads if we need a rotate.

The check for whether a rotate is possible occurs before the
memory legality checks for the integer type. So it's possible we
decide we can use a rotate, but then fail the legality checks. If
that happens we should not fall back to a vector type. This triggers
an assertion in the rotate handling when it finds a vector type
instead of an integer type.

In theory we could use a shufflevector in place of the rotate, but
right now I'd just like to fix the crash.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D108839