Syntactically verify that initializer expressions of '@const' variables and argument expressions to '@const' parameters consist strictly of syntactically-verifiable set of basic values and operations
* `sitofp` signed integer to floating point
* `rint` round floating point to integral
* `bitcast` between integer and floating point
Constant folding `bitcast`s also made it necessary to rewrite constant folding for Nan and inf values, because the old code explicitly checked for `bitcast` intrinsics.
Relying on constant folded `bitcast`s makes the new version much simpler.
It is important to constant fold these intrinsics already in SIL because it enables other optimizations.
