Fix live_ranges idx calculation (https://github.com/Shopify/ruby/pull/353)

forward_pass adjusts the indexes of our opnds to reflect the new
instructions as they are generated in the forward pass. However, we were
using the old live_ranges array, for which the new indexes are
incorrect.

This caused us to previously generate an IR which contained unnecessary
trivial load instructions (ex. mov rax, rax), because it was looking at
the wrong lifespans. Presumably this could also cause bugs because the
lifespan of the incorrectly considered operand idx could be short.

We've added an assert which would have failed on the previous trivial
case (but not necessarily all cases).

Co-authored-by: Matthew Draper <matthew@trebex.net>

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