[NFC][Codegen] shift-amount-mod.ll: drop innermost operation

I have initially added it in for test to display both
whether the binop w/ constant is sinked or hoisted.
But as it can be seen from the 'sub (sub C, %x), %y'
test, that actually conceals the issues it is supposed to test.

At least two more patterns are unhandled:
* 'add (sub C, %x), %y' - D62266
* 'sub (sub C, %x), %y'

llvm-svn: 362295