[mlgo][regalloc] Factor live interval feature calculation

Factoring it out so we can subsequently cache it. This should be a NFC,
however, for the float quantities, we see small errors in the least
significant digits. This is because, before, we were summing up one by
one. Now, we sum up results of sums.

This shouldn't matter for ML, and will require rework when we do
quantization (avoiding floats altogether), but meanwhile, it did require
an update to the reference file used for testing.

The patch also bumps the precision of the variables involved in this, to
reduce the error (note they are casted back to float at the end by the
SET macro, since we only work with float and not double in TF)

Differential Revision: https://reviews.llvm.org/D118659

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