The compiler would produce an `OPCODE_ELSE` with no preceeding `OPCODE_IF`, causing hilarity downstream in the VertexProgram and PixelProgram.

Credit to Dan Sinclair who did all the hard work here.

Bug: None - surfaced when rolling a new version of SPIR-V cross.
Change-Id: I8a937a4d24768dbcc125c3fff834342bd8d0ef29
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/40832Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>