Remove GTF_ADDR_ONSTACK and IsVarAddr.

Remove GTF_ADDR_ONSTACK and IsVarAddr.

IsVarAddr was checking GTF_ADDR_ONSTACK  to determine if
the GT_ADDR node is an address of a local. This change removes both
GTF_ADDR_ONSTACK and  IsVarAddr and uses IsLocalAdrExpr instead.
IsLocalAddrExpr uses opcodes to determine if GT_ADDR node is
a local address.

GTF_ADDR_ONSTACK flag is ancient, added before 2002 so I couldn't find
the checkin that introduced it.

I changed the assert to a check and an assignment since simplifications
inside fgMorphArgs between
https://github.com/dotnet/coreclr/blob/dotnet/coreclr@1a1e4c4d5a8030cb8d82a2e5b06c2ab357b92534/src/jit/morph.cpp#L3709
(which causes https://github.com/dotnet/coreclr/blob/dotnet/coreclr@1a1e4c4d5a8030cb8d82a2e5b06c2ab357b92534/src/jit/morph.cpp#L3057)
and
https://github.com/dotnet/coreclr/blob/dotnet/coreclr@1a1e4c4d5a8030cb8d82a2e5b06c2ab357b92534/src/jit/morph.cpp#L3790
may result in more GT_ADDR nodes recognized by IsLocalAdrExpr.

x86 and x64 pmi frameworks had no code diffs and some gcinfo reductions
(15 methods with gcinfo diffs in x86).

Fixes dotnet/coreclr#22190.

Commit migrated from https://github.com/dotnet/coreclr/commit/4070994640bcc2c4c138b6f695d3fce14ea3efe7