gc: simplify escape analysis recursion

Merge escassign(N, x), escassign(x, N), escexpr(n), and escstmt(n) into esc(n).

1. Renaming
https://golang.org/cl/4917050/diff2/1:2001/src/cmd/gc/esc.c

2. Merging.
https://golang.org/cl/4917050/diff2/2001:4001/src/cmd/gc/esc.c

3. Simplify esccall.
https://golang.org/cl/4917050/diff2/4001:5002/src/cmd/gc/esc.c

R=lvd
CC=golang-dev
https://golang.org/cl/4917050