[wasm] Better support for automated tests in jiterpreter; more opcodes

[kg]

• Rearrange the jiterpreter's compile and enter logic so that we enter a trace immediately after compiling it. If you set the minimum hit count to 1 and disable tiering, this appears to run most automated tests in the jiterpreter, which is useful for verifying opcode implementations and flushing out bugs.
• Move the 'do we need to safepoint after this trace?' logic from the end of enter_jiterpreter into the traces themselves now that traces can perform safepoints. Should improve performance for short traces.
• Implemented a few more opcodes that looked worthwhile based on statistics.

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Issue Details

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• Rearrange the jiterpreter's compile and enter logic so that we enter a trace immediately after compiling it. If you set the minimum hit count to 1 and disable tiering, this appears to run most automated tests in the jiterpreter, which is useful for verifying opcode implementations and flushing out bugs.
• Move the 'do we need to safepoint after this trace?' logic from the end of enter_jiterpreter into the traces themselves now that traces can perform safepoints. Should improve performance for short traces.
• Implemented a few more opcodes that looked worthwhile based on statistics.

Author:	kg
Assignees:	-
Labels:	arch-wasm, area-Codegen-Jiterpreter-mono
Milestone:	-