cmd/compile: fix unnamed parameter handling in escape analysis

For recursive functions, the parameters were iterated using
fn.Name.Defn.Func.Dcl, which does not include unnamed/blank
parameters. This results in a mismatch in formal-actual
assignments, for example,

func f(_ T, x T)

f(a, b) should result in { _=a, x=b }, but the escape analysis
currently sees only { x=a } and drops b on the floor. This may
cause b to not escape when it should (or a escape when it should
not).

Fix this by using fntype.Params().FieldSlice() instead, which
does include unnamed parameters.

Also add a sanity check that ensures all the actual parameters
are consumed.

Fixes #29000

Change-Id: Icd86f2b5d71e7ebbab76e375b7702f62efcf59ae
Reviewed-on: https://go-review.googlesource.com/c/152617


Reviewed-by: Keith Randall <khr@golang.org>