Implement a faster version of function returning check.
Implement a faster version of function returning check.

This commit yields a 40% speed increase on a MIPS binary (nhttpd) with
CPython.

f-prettyland/angr,schieb/angr,chubbymaggie/angr,tyb0807/angr,iamahuman/angr,chubbymaggie/angr,tyb0807/angr,axt/angr,schieb/angr,angr/angr,angr/angr,schieb/angr,axt/angr,axt/angr,chubbymaggie/angr,f-prettyland/angr,tyb0807/angr,iamahuman/angr,iamahuman/angr,f-prettyland/angr,angr/angr