Assembler should track scratch slot types for better type checking

[jannotti]

The following program ought to report a type error at assembly time.

int 1
store 2
load 2
byte "hello"
concat

but it doesn't, because ProgramKnowledge only considers the stack, not scratch slots. But, along the path in a given basic block, knowledge about the types in scratch slows is just as trackable as the stack.

Such a treatment should also be aware that the scratch slots all begin with unit64(0) inside.