cmd/compile: rewrite condselects into doublings and halvings

cmd/compile: rewrite condselects into doublings and halvings

For performance see CL 685676.

This allows something like:
  if y { x *= 2 }

To be compiled to:
  SHLXQ BX, AX, AX

Instead of:
  MOVQ    AX, CX
  SHLQ    $1, CX
  MOVBLZX BL, DX
  TESTQ   DX, DX
  CMOVQNE CX, AX

While ./make.bash uniqued per LOC, there is 2 doublings and 4 halvings.

Change-Id: Ic0727cbf429528a2dbf17cbfc3b0121db8387444
Reviewed-on: https://go-review.googlesource.com/c/go/+/685695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Keith Randall <khr@golang.org>