[RISCV][GISEL] Legalize G_ZEXT, G_SEXT, and G_ANYEXT, G_SPLAT_VECTOR, and...

[RISCV][GISEL] Legalize G_ZEXT, G_SEXT, and G_ANYEXT, G_SPLAT_VECTOR, and G_ICMP for scalable vector types

This patch legalizes G_ZEXT, G_SEXT, and G_ANYEXT. If the type is a
legal mask type, then the instruction is legalized as the element-wise
select, where the condition on the select is the mask typed source
operand, and the true and false values are 1 or -1 (for
zero/any-extension and sign extension) and zero. If the type is a legal integer
or vector integer type, then the instruction is marked as legal.

The legalization of the extends may introduce a G_SPLAT_VECTOR, which
needs to be legalized in this patch for the extend test cases to pass.

A G_SPLAT_VECTOR is legal if the vector type is a legal integer or
floating point vector type and the source operand is sXLen type. This is
because the SelectionDAG patterns only support sXLen typed
ISD::SPLAT_VECTORS, and we'd like to reuse those patterns. A
G_SPLAT_VECTOR is cutom legalized if it has a legal s1 element vector
type and s1 scalar operand. It is...