[Fast-ISel] Don't mark the first use of a remat constant as killed.

When emitting something like 'add x, 1000' if we remat the 1000 then we should be able to
mark the vreg containing 1000 as killed.  Given that we go bottom up in fast-isel, a later
use of 1000 will be higher up in the BB and won't kill it, or be impacted by the lower kill.

However, rematerialised constant expressions aren't generated bottom up.  The local value save area
grows downwards.  This means that if you remat 2 constant expressions which both use 1000 then the
first will kill it, then the second, which is *lower* in the BB will read a killed register.

This is the case in the attached test where the 2 GEPs both need to generate 'add x, 6680' for the constant offset.

Note that this commit only makes kill flag generation conservative.  There's nothing else obviously wrong with
the local value save area growing downwards, and in fact it needs to for handling arbitrarily complex constant expressions.

However, it would be nice if there was a solution which would let us generate more accurate kill flags, or just kill flags completely.

llvm-svn: 236922

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