Implement smart printing of inline asm strings, handling variants and

Implement smart printing of inline asm strings, handling variants and
substituted operands.  For this testcase:

int %test(int %A, int %B) {
  %C = call int asm "xyz $0, $1, $2", "=r,r,r"(int %A, int %B)
  ret int %C
}

we now emit:

_test:
        or r2, r3, r3
        or r3, r4, r4
        xyz r2, r2, r3  ;; look here
        or r3, r2, r2
        blr

... note the substituted operands. :)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25886 91177308-0d34-0410-b5e6-96231b3b80d8