Refactor code. Now the intrinsic lowering pass tries to recycle preexisting

Refactor code.  Now the intrinsic lowering pass tries to recycle preexisting
prototypes, even if they don't precisely match what it would prefer to use.
This fixes: CBackend/2004-02-15-PreexistingExternals.llx compiling it into:

  ltmp_0_30 = memcpy(l14_C, 4u, 17);
  ltmp_1_30 = memcpy(((int *)l27_A), ((unsigned )(long)l27_B), ((int )123u));

instead of:

  ltmp_0_30 = memcpy(l14_C, 4u, 17);
  ltmp_1_27 = l43_memcpy(l27_A, l27_B, 123u);

Which does the wrong thing as you could imagine.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11481 91177308-0d34-0410-b5e6-96231b3b80d8