[PowerPC] Fix assert from machine verify pass that unmatched register class...

[PowerPC] Fix assert from machine verify pass that unmatched register class about fcmp selection in fast-isel

Bad machine code: Illegal virtual register for instruction

function: TestULE
basic block: %bb.0 entry (0x1000a39b158)
instruction: %2:crrc = FCMPUD %1:vsfrc, %3:f8rc
operand 1: %1:vsfrc

Fix assert about missing match between fcmp instruction and register class. 
We should use vsx related cmp instruction xvcmpudp instead of fcmpu when vsx is opened.

add -verifymachineinstrs option into related test cases to enable the verify pass.


Differential Revision: https://reviews.llvm.org/D55686

llvm-svn: 350685