HLSL is very fussy about fallthrough in switch blocks, so promote
Unreachable blocks to breaks if they are inside a switch construct.
Some false positives are possible in weird multi-break scenarios, but
this is benign.
We have been interchanging spv and SPIRV_Cross_ for a while, which
causes weirdness since we don't explicitly ban SPIRV_Cross identifiers,
as these identifiers are generally used for interface variable
workarounds.