[RISCV] Add support for Zihintpause extention

Add support for the 'pause' hint instruction as an alias for
'fence w, 0'. To do this allow the 'fence' operands pred and succ
to be set to 0 (the empty set). This will also allow future hints
to be encoded as 'fence 0, <x>' and 'fence <x>, 0'.

This patch revised from @mundaym's D93019.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D117789