memnode.cpp,100 assert(consistent,"adr_type must match alias idx") triggered during 1st pass of IGVN

This is a defect discovered by the customer using -XX:+CompileTheWorld option. It has never occured in production. Customer can reproduce this using their own testing framework that allows to compile certain specific inline tree. Customer proposes a proactive fix for this issue (see suggested fix)

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I have encountered a C2 defect related to IGVN and address types. My
fix is scheduled for a release, so I'd like to get a review from Sun.

Symptom in v1.5.0_08:
 memnode.cpp,100 assert(consistent,"adr_type must match alias idx")

I looked for a possible fix in 6.0, but did not notice anything, and
didn't see a report in the Sun bug database. I haven't been able to
test with 6.0 yet.

The assert occurs during the first pass of IGVN.

Before IGVN we have an AddP feeding a LoadP.

958 AddP === _ (NULL) 955 (LoadKlass) 955 (LoadKlass) 957 (ConI)
  [24700958] Klass:klass java/lang/Object: 02dcdba0:NotNull+280 *

959 LoadP === _ (NULL) 1711 (MergeMem) 958 (AddP)
  @klass java/lang/Object: 02dcdba0:NotNull+280 *, idx=22;
  [24700959] Oop:java/lang/Object:NotNull *

During IGVN, the base address of 958-AddP is reduced to a constant:

1714 ConP === 0 (Root)
  [45801714] Klass:precise klass java/lang/String:
02dc52d8:Constant:exact *

IGVN pops 959-LoadP before 958-AddP and performs
MemNode::Ideal_common. Inside MemNode::Ideal_common we have:

const Type *t_adr = phase->type( address ); <= (klass Object+280)
const TypePtr *adr_t = adr_type(); <= (klass String:exact+280)
consistent = phase->C->must_alias(adr_t, alias_idx ); <= false

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Analysis:

The Object.hashcode native intrinsic loads a vtable entry from the
object's klass using the inexact klass type "klass java/lang/Object
+280".

Type inference later improves the precision of the klass type to
"klass java/lang/String+280". However, flatten_alias_type() ASSUMES
that all klass fields beyond the initial Klass fields refer to static
fields and gives them a unique alias index. But in this case, both
types refer to the same data so should have the same alias index.

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