...information.

For both multiply and divide the old code was writing a long-hand

reduced version of the math without any of the special handling of inf

and NaN recommended by the standard here. Instead of putting more

complexity here, this change does what GCC does which is to emit

a libcall for the fully general case.

However, the old code also failed to do the proper minimization of the

set of operations when there was a mixed complex and real operation. In

those cases, C provides a spec for much more minimal operations that are

valid. Clang now emits the exact suggested operations. This change isn't

*just* about performance though, without minimizing these operations, we

again lose the correct handling of infinities and NaNs. It is critical

that this happen in the frontend based on assymetric type operands to

complex math operations.

The performance implications of this change aren't trivial either. I've

run a set of benchmarks in Eigen, an open source mathematics library

that makes heavy use of complex. While a few have slowed down due to the

libcall being introduce, most sped up and some by a huge amount.

TODO: In order to make all of this work, also match the algorithm in the

constant evaluator to the one in the runtime library. Currently it is a broken

port of the simplifications from C's Annex G to the long-hand formulation of

the algorithm.

Splitting this patch up is very hard because none of this works without

the AST change to preserve non-complex operands. Sorry for the enormous

change.