target/ppc: Fix for optimized vsl/vsr instructions

In previous implementation, invocation of TCG shift function could request
shift of TCG variable by 64 bits when variable 'sh' is 0, which is not
supported in TCG (values can be shifted by 0 to 63 bits). This patch fixes
this by using two separate invocation of TCG shift functions, with maximum
shift amount of 32.

Name of variable 'shifted' is changed to 'carry' so variable naming
is similar to old helper implementation.

Variables 'avrA' and 'avrB' are replaced with variable 'avr'.

Fixes: 4e6d0920
Reported-by: "Paul A. Clark" <pc@us.ibm.com>
Reported-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Suggested-by: Aleksandar Markovic <aleksandar.markovic@rt-rk.com>
Signed-off-by: Stefan Brankovic <stefan.brankovic@rt-rk.com>
Message-Id: <1570196639-7025-2-git-send-email-stefan.brankovic@rt-rk.com>
Tested-by: Paul A. Clarke <pc@us.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>