[RISCV][VP] Lower VP_MERGE to RVV instructions

This patch adds lowering of the llvm.vp.merge.* intrinsic
(ISD::VP_MERGE) to RVV vmerge/vfmerge instructions. It introduces a
special pseudo form of vmerge which allows a tied merge operand,
allowing us to specify the tail elements as being equal to the "on
false" operand, using a tied-def constraint and a "tail undisturbed"
policy.

While this strategy allows us to often lower the intrinsic to just one
instruction, it may be less efficient in fixed-vector types as the
number of tail elements may extend far beyond the length of the fixed
vector. Another strategy could be to use a vmerge/vfmerge instruction
with an AVL equal to the length of the vector type, and manipulate the
condition operand such that mask elements greater than the operation's
EVL are false.

I've also observed inefficient codegen in which our 'VF' patterns don't
match raw floating-point SPLAT_VECTORs, which occur in scalable-vector
code.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D117561