Allow taking Matrix slices without an extra allocation (#56236)
Since changing Array to use Memory as the backing, we had the option of
making non-Vector arrays more flexible, but had instead preserved the
restriction that they must be zero offset and equal in length to the
Memory. This results in extra complexity, restrictions, and allocations
however, but doesn't gain many known benefits. Nanosoldier shows a
decrease in performance on linear eachindex loops, which we theorize is
due to a minor failure to CSE before SCEV or a lack of NUW/NSW on the
length multiplication calculation.
