[mips] Show an error if register number is out of range

Current code does not check that a register number is in the 0-31 range.
Sometimes the parser checks that later for some kinds of instructions,
but that leads to unclear / incorrect error messages like that:

  % cat test.s
  .text
  lb $4, 8($32)

  % llvm-mc test.s -triple=mips64-unknown-linux
  test.s:2:10: error: expected memory with 16-bit signed offset
    lb $4, 8($32)
           ^

Sometimes the parser just crashes:

  % cat test.s
  .text
  lw  $4, 8($32)

  % llvm-mc test.s -triple=mips64-unknown-linux

This patch resolves the problem by checking that register number after
'$' sign is in the 0-31 range. If the number is out of the range the
parser shows the `invalid register number` error, but treats invalid
register number as a normal one to continue parsing and catch other
possible errors.

Differential Revision: https://reviews.llvm.org/D45919

llvm-svn: 330732