Fix asserts that would trip during emit, which we only use to emit textual
representation of generated asm to stdout. Useful for debugging.

Bug: b/144688789
Change-Id: I42d60da042fa04bf6afc9035b2892249e5e46c59
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38475Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>