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Issue Description:

I was playing around with one of the benchmark game programs and ran into a few performance edge cases.

The first was that marking a method as public caused inlining behaviour to change even in `-wmo` mode. Compare the effects of marking `dot` as `public`:

https://godbolt.org/z/WPZtHx (internal) vs https://godbolt.org/z/QcurxC (public). When `dot` is marked as public, `addingProduct` does not get inlined.

The second issue I noticed was that calling `SIMD4<Double>.init(_ value: SIMD4<Double>)` causes a bunch of calls to:

(generic specialization <Swift.Double> of generic partial specialization <serialized, Signature = @escaping @convention(thin) @convention(method) <A><A1 where A == Swift.Double, A1: Swift.BinaryFloatingPoint> (@in_guaranteed A1, @unowned @thick Swift.Double.Type) -> (@out Swift.Double, @unowned Swift.Bool)> of static (extension in Swift):Swift.BinaryFloatingPoint._convert<A where A1: Swift.BinaryFloatingPoint>(from: A1) -> (value: A, exact: Swift.Bool))

even though it should be a no-op. This can easily be observed by e.g. changing line 57 from `let posi = bodyi.pos` to `let posi = Vec4(bodyi.pos)`.

The third is that changing `System` from a `struct` to a `class` (https://godbolt.org/z/tJSudD) increases execution time from 2.56 to 3.43 on my computer, when it should have no change on the behaviour.

The test case:

main.swift with compile flags `-Ounchecked -wmo -Xllvm -unroll-count=5 -Xllvm -unroll-threshold=500 -Xcc -march=native -Xcc -ffast-math -Xcc -mavx2`

This was all tested with Swift 5.2 dev, commit 2007515