[RISCV] Add some missing expansions for floating-point intrinsics

A number of intrinsics, such as llvm.sin.f32, would result in a failure to 
select. This patch adds expansions for the relevant selection DAG nodes, as 
well as exhaustive testing for all f32 and f64 intrinsics.

The codegen for FMA remains a TODO item, pending support for the various 
RISC-V FMA instruction variants.

The llvm.minimum.f32.* and llvm.maximum.* tests are commented-out, pending 
upstream support for target-independent expansion, as discussed in 
http://lists.llvm.org/pipermail/llvm-dev/2018-November/127408.html.

Differential Revision: https://reviews.llvm.org/D54034
Patch by Luís Marques.

llvm-svn: 346034