cmd/compile: remove superfluous signed right shift used for signed division by 2

A signed right shift before an unsigned right shift by register width-1
(extracts the sign bit) is superflous.

trigger counts during ./make.bash
 0   (Rsh8U  (Rsh8  x _) 7  ) -> (Rsh8U  x 7 )
 0   (Rsh16U (Rsh16 x _) 15 ) -> (Rsh16U x 15)
 2   (Rsh32U (Rsh32 x _) 31 ) -> (Rsh32U x 31)
 251 (Rsh64U (Rsh64 x _) 63 ) -> (Rsh64U x 63)

Changes the instructions generated on AMD64 for x / 2 where
x is a signed integer from:

 MOVQ    AX, CX
 SARQ    $63, AX
 SHRQ    $63, AX
 ADDQ    CX, AX
 SARQ    $1, AX

to:

 MOVQ    AX, CX
 SHRQ    $63, AX
 ADDQ    CX, AX
 SARQ    $1, AX

Change-Id: I86321ae8fc9dc24b8fa9eb80aa5c7299eff8c9dc
Reviewed-on: https://go-review.googlesource.com/115956


Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>