VMSLG has three variants on z14 and later machines. These variants are used in "limbified" squaring:
VMSLEG: Even Shift Indication -- the even-indexed intermediate result is doubled
VMSLOG: Odd Shift Indication -- the odd-indexed intermediate result is doubled
VMSLEOG: Even and Odd Shift Indication -- both intermediate results are doubled
Limbified squaring is very useful for high performance cryptographic algorithms, such as
elliptic curve. This change allows these instructions to be used in Go assembly.

Change-Id: Iaad577b07320205539f99b3cb37a2a984882721b
Reviewed-on: https://go-review.googlesource.com/c/145180Reviewed-by: Michael Munday <mike.munday@ibm.com>