cmd/compile: do more type conversion inline

cmd/compile: do more type conversion inline

The code to do the conversion is smaller than the
call to the runtime.
The 1-result asserts need to call panic if they fail, but that
code is out of line.

The only conversions left in the runtime are those which
might allocate and those which might need to generate an itab.

Given the following types:
  type E interface{}
  type I interface { foo() }
  type I2 iterface { foo(); bar() }
  type Big [10]int
  func (b Big) foo() { ... }

This CL inlines the following conversions:

was assertE2T
  var e E = ...
  b := i.(Big)
was assertE2T2
  var e E = ...
  b, ok := i.(Big)
was assertI2T
  var i I = ...
  b := i.(Big)
was assertI2T2
  var i I = ...
  b, ok := i.(Big)
was assertI2E
  var i I = ...
  e := i.(E)
was assertI2E2
  var i I = ...
  e, ok := i.(E)

These are the remaining runtime calls:

convT2E:
  var b Big = ...
  var e E = b
convT2I:
  var b Big = ...
  var i I = b
convI2I:
  var i2 I2 = ...
  var i I = i2
assertE2I:
  var e E = ...
  i := e.(I)
assertE2I2:
  var e E = ...
  i, ok := e.(I)
assertI2I:
  var i I = ...
  i2 := i.(I2)
assertI2I2:
  var i I = ...
  i2, ok := i.(I2)

Fixes #17405
Fixes #8422

Change-Id: Ida2367bf8ce3cd2c6bb599a1814f1d275afabe21
Reviewed-on: https://go-review.googlesource.com/32313
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>