[WebAssembly] Allow AsmTypeCheck detect multiple errors in function

This allows multiple errors to be reported within a function, rather
than returning on the first error and not looking at the rest of the
function.

I think the rationale for the previous behavior was that upon
encountering the first error, the value stack was not in the correct
status anymore and the rest of the function checking was not very
meaningful. But this patch makes the instruction push the correct result
type upon its completion, so the while the possibility of previous error
affecting later instructions is not zero, I think this can be more
helpful to assembly hand-writers. This also allows us to write multiple
error test cases without creating as many functions.

This is what Wabt and Binaryen wast parsers do as well.

Also this makes sure we return a value (true/false) within an `if` for
each instruction, removing the need for the long `if`-`else if`-
`else if` chain and making them all just `if`s. I also added a newline
between each `if`s, which I feel is easier to read.

Author: aheejin