[VectorCombine] Add tests for unspeculatable VP binops. NFC

The current test cases to guard against speculative execution can actually be
safely speculated because the denominator is known to be not 0 or -1, and
isSafeToSpeculativelyExecuteWithOpcode will account for this. This adds some
more test cases and rejigs some existing ones to use an unknown variable
instead.

Author: lukel97