compiler: permit inlining temporary statements and references

This increases the number of inlinable functions from 439 to 455.
An example is math/bits.Mul32, which uses temporaries to handle the
tuple assignment.

Change-Id: I3a5c745a826de28af4269163bb3745003780499e
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/180837
Reviewed-by: Cherry Zhang <cherryyz@google.com>
8 files changed