Recommit Eric's code to validate ASM string's constraints and modifiers.

This code checks the ASM string to see if the output size is able to fit within
the variable specified as the output. For instance, scalar-to-vector conversions
may not really work. It's on by default, but can be turned off with a flag if
you think you know what you're doing.

This is placed under a flag ('-Wasm-operand-widths') and flag group ('-Wasm').

<rdar://problem/12284092>

llvm-svn: 166737