Under certain conditions, the Dest value of InstX86Movd doesn't get a
register allocated to it. This is unlikely the right fix, as it's
unidiomatic, but pending further investigation, this seems to do the
trick.

Bug: b/145529686
Change-Id: I5b5d5148aef04ebac957d5941779d5cd8e544098
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38871
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>