delete a loop that just generates dead code. In an example

delete a loop that just generates dead code.  In an example
like this:

void *test(long N) {
  return new int[N][42][42];
}

the loop generates two dead mul instructions:

  %tmp = load i64* %N.addr                        ; <i64> [#uses=2]
  %0 = mul i64 %tmp, 7056                         ; <i64> [#uses=1]
  %1 = mul i64 %tmp, 42                           ; <i64> [#uses=1]
  %2 = mul i64 %1, 42                             ; <i64> [#uses=0]
  %call = call noalias i8* @_Znam(i64 %0)         ; <i8*> [#uses=1]

The scale of these multiplies is already handled by the typesize stuff.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@108884 91177308-0d34-0410-b5e6-96231b3b80d8