cmd/compile: split n.Noescape() into separate uses
n.Noescape() was overloaded for two uses: (1) to indicate a function
was annotated with //go:noescape, and (2) to indicate that certain
temporary allocations don't outlive the current statement.
The first use case is redundant with n.Func.Pragma&Noescape!=0, which
is the convention we use for checking other function-level pragmas.
The second use case is better served by renaming "Noescape" to
"Transient".