[RISCV] Add overlapping constraints label

The RISCV-V-Spec[1] describes which RVV instructions are allowed to overlap between source operands and destination operands. Currently, most of these instructions are constrained with early-clobber.

This patch labels these instructions with TargetOverlapConstraintType in pseudo to indicate that these instructions can overlap.

The value of TargetOverlapConstraintType is:

1 -> default
2 -> narrow case
3 -> widen case

This information is useful for future optimization passes.

[1] https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#52-vector-operands

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D157588

Author: BeMg