Added tracking to types and validated certain instructions.

Added tracking to types and validated certain instructions.

We need to know how to generate correct SPIRV for cases like
OpConstant %int64 42 since the current parser will encode the 42 as a
32-bit value incorrectly.

This change is the first of a pair. This one tracks types, and makes
sure that OpConstant and OpSpecConstant are only ever called with
Integer or Float types, and OpSwitch is only called with integer
generating values.