Implement `st.lclFld` decomposition and fix #6778.

The problem that was blocking both of these issues was an issue in
liveness when analyzing `st.lclFld` nodes that store to an unpromoted
long-typed local variable or any long-typed field. The latter case--a
`st.lclFld` node that targets a long-typed field--is easy to handle.
Such stores are decomposed from:

```
         /--*  t0 int
         +--*  t1 int
    t2 = *  gt_long long

         /--*  t2
         *  st.lclFld long V00
```

To:

```
         /--* t0
         *  st.lclFld int V00 [+0]

         /--* t1
         *  st.lclFld int V00 [+4]
```

The two `st.lclFld` nodes that this transformation generates are marked
with `GTF_VAR_USEASG` to indicate that they are partial (rather than
complete) defs. This is necessary in order to correctly calculate
liveness.

Before this change, stores to unpromoted long-typed local variables
were also decomposed in the above fashion. These local vars can arise in
a number of situations:
- vars that are used to store multi-reg args or return values
- vars that represent long-typed fields of promoted struct-typed vars
- long-typed vars that are marked as unenregisterable before decomp

Unfortunately, the decomposition given above differs in its liveness
semantics when compared to the original IR: in the original IR, the
`st.lclVar` is considered a plain def of its destination, while in
the decomposed IR, each `st.lclFld` is considered both a partial def
and a use of its destination. This difference--namely, that in the
first case the destination is not used at the def and in the second
case it is--causes problems for any analysis that needs to take into
account whether or not a variable is used at a def (e.g. dead store
removal, live-in sets, etc.). In order to retain the original
semantics without complicating the IR, stores to unpromoted, long-typed
local variables are not decomposed. These stores are instead handled
during code generation.

1 files changed, 0 insertions, 6 deletions

diff --git a/tests/issues.targets b/tests/issues.targets
index 41be913202..45cb5a98b4 100644
--- a/tests/issues.targets
+++ b/tests/issues.targets
@@ -239,12 +239,6 @@
     <!-- The following x86 failures only occur with RyuJIT/x86 -->
 
     <ItemGroup Condition="'$(XunitTestBinBase)' != '' and '$(BuildArch)' == 'x86'">
-        <ExcludeList Include="$(XunitTestBinBase)\JIT\jit64\eh\basics\loopEH\loopEH.cmd">
-            <Issue>6778</Issue>
-        </ExcludeList>
-        <ExcludeList Include="$(XunitTestBinBase)\JIT\Methodical\int64\superlong\_il_relsuperlong\_il_relsuperlong.cmd">
-            <Issue>6778</Issue>
-        </ExcludeList>
         <ExcludeList Include="$(XunitTestBinBase)\GC\Scenarios\DoublinkList\dlstack\*">
             <Issue>6553</Issue>
         </ExcludeList>