[RISCV][NFC] Define and use the new CA instruction format

The RISC-V ISA manual was updated on 2018-11-07 (commit 00557c3) to define a 
new compressed instruction format, RVC format CA (no actual instruction 
encodings were changed). This patch updates the RISC-V backend to define the 
new format, and to use it in the relevant instructions.

Differential Revision: https://reviews.llvm.org/D54302
Patch by Luís Marques.

llvm-svn: 347043