cmd/compile: do not change field offset in ABI analysis

Currently, the ABI analysis assigns parameter/result offsets
to the fields of function *Type. In some cases, we may have
an ABI0 function reference and an ABIInternal reference share
the same function *Type. For example, for an ABI0 function F,
"f := F" will make f and (ABI0) F having the same *Type. But f,
as a func value, should use ABIInternal. Analyses on F and f will
collide and cause ICE.

Also, changing field offsets in ABI analysis has to be done very
carefully to avoid data races. It has been causing
trickiness/difficulty.

This CL removes the change of field offsets in ABI analysis
altogether. The analysis result is stored in ABIParamAssignment,
which is the only way to access parameter/result stack offset now.

Fixes #47317.
Fixes #47227.

Change-Id: I23a3e081a6cf327ac66855da222daaa636ed1ead
Reviewed-on: https://go-review.googlesource.com/c/go/+/336629


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Than McIntosh <thanm@google.com>