JIT: bail on flipping post-layout JTRUE when reversal needs new IR

[Copilot]

optOptimizePostLayout runs after LSRA and called gtReverseCond, which can fall back to wrapping the operand in a fresh GT_EQ(tree, 0). The new GT_INT_CON child is never inserted into the LIR, tripping found use of a node that is not in the LIR sequence in checked builds (reported by Fuzzlyn on linux-arm32). The fallback was also reached whenever LSRA inserted a GT_COPY/GT_RELOAD on top of the JTRUE operand, since the wrapper is not itself a comparison.

Description

• gentree.cpp / compiler.h — Extract gtTryReverseCond(GenTree*) -> bool that reverses compares, JCC/SETCC, JCMP/JTEST, and integral constants in place. gtReverseCond becomes a thin wrapper that adds the GT_EQ(tree, 0) fallback when in-place reversal isn't possible, preserving semantics for all existing callers.
• optimizer.cpp — In optOptimizePostLayout, peel GT_COPY/GT_RELOAD off the GT_JTRUE operand via gtSkipReloadOrCopy(), then use gtTryReverseCond and skip flipping the block if it returns false rather than mutating the LIR.

GenTree* cond = test;
if (test->OperIs(GT_JTRUE))
{
    cond = test->gtGetOp1()->gtSkipReloadOrCopy();
}

if (!gtTryReverseCond(cond))
{
    continue;
}

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