[RISCV] Implement RISCV ABI lowering

RISCVABIInfo is implemented in terms of XLen, supporting both RV32 and RV64. 
Unfortunately we need to count argument registers in the frontend in order to 
determine when to emit signext and zeroext attributes. Integer scalars are 
extended according to their type up to 32-bits and then sign-extended to XLen 
when passed in registers, but are anyext when passed on the stack. This patch 
only implements the base integer (soft float) ABIs.

For more information on the RISC-V ABI, see [the ABI 
doc](https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md), 
my [golden model](https://github.com/lowRISC/riscv-calling-conv-model), and 
the [LLVM RISC-V calling convention 
patch](https://reviews.llvm.org/D39898#2d1595b4) (specifically the comment 
documenting frontend expectations).

Differential Revision: https://reviews.llvm.org/D40023

llvm-svn: 322494