[RISCV] Mark FREM as Expand

[RISCV] Mark FREM as Expand

Mark the FREM SelectionDAG node as Expand, which is necessary in order to
support the frem IR instruction on RISC-V. This is expanded into a library
call. Adds the corresponding test. Previously, this would have triggered an
assertion at instruction selection time.

Differential Revision: https://reviews.llvm.org/D54159
Patch by Luís Marques.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@346958 91177308-0d34-0410-b5e6-96231b3b80d8