[RISCV] Codegen for i8, i16, and i32 atomicrmw with RV32A

[RISCV] Codegen for i8, i16, and i32 atomicrmw with RV32A

Introduce a new RISCVExpandPseudoInsts pass to expand atomic
pseudo-instructions after register allocation. This is necessary in order to
ensure that register spills aren't introduced between LL and SC, thus breaking
the forward progress guarantee for the operation. AArch64 does something
similar for CmpXchg (though only at O0), and Mips is moving towards this
approach (see D31287). See also [this mailing list
post](http://lists.llvm.org/pipermail/llvm-dev/2016-May/099490.html) from
James Knight, which summarises the issues with lowering to ll/sc in IR or
pre-RA.

See the [accompanying RFC
thread](http://lists.llvm.org/pipermail/llvm-dev/2018-June/123993.html) for an
overview of the lowering strategy.

Differential Revision: https://reviews.llvm.org/D47882

llvm-svn: 342534