[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.

llvm-svn: 346958