[Hexagon] Check for offset overflow when reserving scavenging slots

[Hexagon] Check for offset overflow when reserving scavenging slots

Scavenging slots were only reserved when pseudo-instruction expansion in
frame lowering created new virtual registers. It is possible to still
need a scavenging slot even if no virtual registers were created, in cases
where the stack is large enough to overflow instruction offsets.

llvm-svn: 277355