During legalisation we can fold / combine `sext` followed by a widening
via `zext`. Unfortunately, this yields a new result register with no
SPIRV Type, which leads to incorrect behaviour during post legalisation
when we end up deducing the (narrower) type from the operand. This patch
corrects the behaviour in that it ensures that we use the (widened) type
of the result to yield the SPIRV Type for the register.
e531f48fc4