RFR[XS]: 8238272: Eliminate cast_from_oop to narrowOop*

[Kim Barrett]

Please review this change to ObjArrayKlass::oop_oop_iterate_range to
eliminate a conditional cast_from_oop to either either narrowOop* or
oop*.  Casting an oop to either of those types seems pretty
questionable, and is not actually necessary here.

(This appears to be the *only* place where cast_from_oop<narrowOop*>()
is used.  There are other casts to oop* that should be examined.)

This change has the additional minor benefit that it allows the
compiler to eliminate the lower bounds clamp when start is a constant
zero, and eliminates the conditional test for zero when it isn't a
constant.

CR:
https://bugs.openjdk.java.net/browse/JDK-8238272

Webrev:
https://cr.openjdk.java.net/~kbarrett/8238272/open.00/

Testing:
mach5 tier1-5
Compared gcc generated code for linux-x64 with and without the change
for calls with and without a literal 0 start index.