[NFC] [Power] Fix instruction format for xsrqpi

xsrqpi is currently using Z23Form_1.
The instruction format is xsrqpi R,VRT,VRB,RMC.
Rathar than bits 11-15 being used for FRA, it should have
bits 11-14 reserved and bit 15 for R. This patch adds a new
class Z23Form_4 to fix the instruction format.

Differential Revision: https://reviews.llvm.org/D46761

llvm-svn: 332253