Request for reviews (S): 7000491: assert(false) failed: should be optimized out in SharedRuntime::g1_wb_pre

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/7000491/webrev

Fixed 7000491: assert(false) failed: should be optimized out in SharedRuntime::g1_wb_pre

Wrong value type is used for NULL store when clearing
the detail message of the preallocated exception object
for inlined throw.
The passed value type was the exception object oop which
is constant and as result the load of original value in
g1 pre barrier code was replaced with this constant oop.
As result the check of original value for NULL was
incorrectly eliminated.

Use String type as value type for the detail message zeroing.
Verified with generated code for spec.jbb.validity.PepTest::testDiv