[llvm] [GISEL] Add G_SPLAT_VECTOR_PARTS to represent 64-bit splat vectors on… (PR #86970)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> I assume that this again for patterns, but `G_SPLAT_VECTOR(G_MERGE_VALUES(lower, upper))` is the same?!?

This is for patterns. There are no patterns for SPLAT_VECTOR with an i64 operand on RV32.

https://github.com/llvm/llvm-project/pull/86970