[RISCV] Use register X0 (ZERO) for constant 0

The obvious approach of defining a pattern like the one below actually doesn't
work:
`def : Pat<(i32 0), (i32 X0)>;`

As was noted when Lanai made this change (https://reviews.llvm.org/rL288215),
attempting to handle the constant 0 in tablegen leads to assertions due to a
physical register being used where a virtual register is expected.

llvm-svn: 318738