Wasm backend: ensure correct translation of Wasm_ir.global_(get|set)

[pkel]

While working on a different issue, I skimmed the following lines. The use of LocalGet / LocalSet where we want to handle global variables is almost certainly a bug.

qcert/compiler/wasm/wasm_ir.ml

Lines 261 to 265 in 9969786

	let local_get i _ = GetLocal (Int32.of_int i @@ no_region)
	let local_set i _ = SetLocal (Int32.of_int i @@ no_region)
	let local_tee i _ = TeeLocal (Int32.of_int i @@ no_region)
	let global_get x ctx = GetLocal (global_to_spec ctx x)
	let global_set x ctx = SetLocal (global_to_spec ctx x)

I want to add tests and fix the bug.