[RISCV] Implement c.lui immediate operand constraint

Implement c.lui immediate constraint to [1, 31] and [0xfffe0, 0xfffff].
The RISC-V ISA describes the constraint as [1, 63], with that value
being loaded in to bits 17-12 of the destination register and sign extended
from bit 17. Therefore, this 6-bit immediate can represent values in the
ranges [1, 31] and [0xfffe0, 0xfffff].

Differential Revision: https://reviews.llvm.org/D42834

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325792 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed