AMDGPU: Change API for checking for exec modification

Invert the name and return value to better reflect the imprecise
nature.

Force passing in the DefMI, since it's known in the 2 users and could
possibly fail for an arbitrary vreg.

Allow specifying a specific user instruction. Scan through use
instructions, instead of use operands. Add scan thresholds instead of
searching infinitely.

Stop using a set to track seen uses. I didn't understand this usage,
or why it would not check the last use. I don't think the use list has
any particular order.

llvm-svn: 363675