Text syntax for SIMD lane index vs memory index operands

[AndrewScheidecker]

This proposal adds a memidx to the syntax for load/store instructions: i32.load 1 loads from memory index 1.

The SIMD proposal adds instructions with a laneidx immediate that load or store a single lane of a vector: v128.load8_lane 1 loads a scalar and replaces lane index 1 in a v128 operand.

If the proposals are merged, it's unclear whether the 1 in v128.load8_lane 1 should be parsed as a memory or lane index. It's possible to disambiguate it by saying that if there's a single index, it's the lane index, and if there are two indices, it's the memory index followed by the lane index.

Alternatively, we could require that memory index immediates are wrapped in a (memory ..) tag like in an active data segment definition. i.e.:

• v128.load8_lane 2 would load a scalar from memory 0 and replace lane index 2.
• v128.load8_lane (memory 1) would be malformed due to missing a lane index.
• v128.load8_lane (memory 1) 2 would load a scalar from memory 1 and replace lane index 2.

[AndrewScheidecker]

Oops, this is a dupe of #17.