compiler: support new language constructs in escape analysis

compiler: support new language constructs in escape analysis

Previous CLs add new language constructs in Go 1.17, specifically,
unsafe.Add, unsafe.Slice, and conversion from a slice to a pointer
to an array. This CL handles them in the escape analysis.

At the point of the escape analysis, unsafe.Add and unsafe.Slice
are still builtin calls, so just handle them in data flow.
Conversion from a slice to a pointer to an array has already been
lowered to a combination of compound expression, conditional
expression and slice info expressions, so handle them in the
escape analysis.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/339671