Add initial support for immediates. This allows us to compile this:

Add initial support for immediates.  This allows us to compile this:

int %rlwnm(int %A, int %B) {
  %C = call int asm "rlwnm $0, $1, $2, $3, $4", "=r,r,r,n,n"(int %A, int %B, int 4, int 17)
  ret int %C
}

into:

_rlwnm:
        or r2, r3, r3
        or r3, r4, r4
        rlwnm r2, r2, r3, 4, 17    ;; note the immediates :)
        or r3, r2, r2
        blr

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25955 91177308-0d34-0410-b5e6-96231b3b80d8