[llvm] [RISCV][GISEL] instruction-select for G_SPLAT_VECTOR (PR #111193)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> We want the constant to be folded as an operand of the final instruction. It needs to be visible to a complex render function that is called by the isel table. selectRISCVIConstant creates the constant in a separate register.

So the issue is vector constants look different in the DAG and GISel? Could we have the pattern importer start to recognize the G_SPLAT_VECTOR case, instead of the G_BUILD_VECTOR of constants? 

https://github.com/llvm/llvm-project/pull/111193