The insertion point of COPY isn't always optimal and could eventually
lead to a worse block layout, see the regression test in the first
commit.
This change affects many architectures but the amount of total
instructions in the test cases seems too be slightly lower.
This is a reduced version of one of the tests that was broken by the
original commit of D154281 "[CodeGen] Store SP adjustment in
MachineBasicBlock. NFCI.".
Differential Revision: https://reviews.llvm.org/D155471
