Emit POP using local stack pointer

[Giulio2002]

Summary

• generate the POP body directly in the shaped POP case
• reuse the local s := interp.Stack already needed for the stack check
• avoid going back through interp.Stack.top-- in the inlined opPop body

Benchmark

Focused opcode benchmark against origin/master baseline:

• Opcode/PUSH1_POP: 6.392ms -> 6.187ms, -3.20%
• nearby controls (ADD, DUP1_POP, SWAP1) were statistically unchanged

Verification

• go generate ./vm
• git diff --check
• go test ./vm -count=1
• go test ./... -count=1
• GEVM_TEST_RUNNER=opcode go test ./... -count=1
• go test ./tests/bench -run '^$' -bench '^BenchmarkOpcode/(PUSH1_POP|DUP1_POP|ADD|SWAP1)$' -benchmem -count=10