In several heavily-used paths, prove is checking for the existence of a zero-valued constant, and creating it if not found:

• After initialisation, prove ranges through all blocks and values, checking for the zero constant at every Value until it finds one. A similar check is also done here for every string len, slice len, and slice cap op.
• factTable.isNonNegative contains a similar check needed to take advantage of the poset.

I’d like to clean this up in two ways:

1. Preempt all these checks by finding or creating the zero constant Value when the factsTable is initialised on entry to prove().
2. Use a method from func.go, func.ConstInt64(), that takes care of locating an extant value or creating one if necessary.

(On mobile, will fix formatting and add details/links when back to keyboard)

cc/ @rasky @josharian @mvdan