call gtFoldExpr after fgMorphExpandInstanceField

[EgorBo]

Fixes #103577

it's a pretty annoying assert that we should probably just delete - gtMarkAddrMode may hit that assert when it encounters cns + cns like foldable trees in tier0 (with opts) or tier1, because it does its own foldings and after that runs some validations

fgMorphExpandInstanceField may return:

[000023] -----------                         *  ADD       long
[000003] H----------                         +--*  CNS_INT(h) long   0x1c880109658 static Fseq[foo]
[000022] -----------                         \--*  CNS_INT   long   4

[EgorBo]

/azp run runtime-coreclr jitstress

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[EgorBo]

PTAL @jakobbotsch @dotnet/jit-contrib quick fix for an assert to unblock jitstress