[Umbrella] Switch stmt CIRGen issues

[bcardosolopes]

There are many current issues with switch statements, mostly regarding all weird supported combinations. This issue is aggregating all of them in order to facilitate folks to coordinate and execute.

• Assertion failure on switch statement with code outside a case: Assertion failure on switch statement with code outside a case  #521
• Assertion failure on switch statement with non-block substatement: Assertion failure on switch statement with non-block substatement #520
• Assertion failure on switch statement with case label in nested block: Assertion failure on switch statement with case label in nested block #522
• Miss destruction in switch statements: Miss destruction in switch statements #587
• Module verification error of switch statements: Module verification error of switch statements #588
• Incorrect terminator generation: Incorrect terminator generation #602

CC: @wenpen @piggynl @gitoleg