[RISCV] Optimize floating-point "dominant value" BUILD_VECTORs

[RISCV] Optimize floating-point "dominant value" BUILD_VECTORs

This patch aims to improve the performance of BUILD_VECTORs which are
identified as containing a dominant element. Given that most
floating-point constants themselves require a load from the constant
pool, it was possible for the optimization to actually increase the
number of individual loads on small vectors. The exception is the zero
constant -- +0.0 -- which can be materialized efficiently.

While this optimization could do with a proper cost model to weigh the
benfits of a single vector load vs. the manipulation of individual
elements -- even for integer vectors which often require several
instructions to materialize -- without a concrete RVV implementation to
work with any heuristic is likely to be both more obtuse and inaccurate.

Until then, this patch fixes at least one known obvious deficiency.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106963