This CL merges the dev.regabi branch to the master branch.
In the dev.regabi branch we have refactored the compiler, and laid
some preliminary work for enabling a register-based ABI (issue #40724),
including improved late call/return lowering, improved ABI wrapper
generation, reflect call prepared for the new ABI, and reserving
special registers in the internal ABI. The actual register-based ABI
has not been enabled for the moment. The ABI-related changes are behind
GOEXPERIMENT=regabi and currently off by default.
Updates #40724, #44222.
Fixes #44224.
Change-Id: Id5de9f734d14099267ab717167aaaeef31fdba70