[tablegen] Avoid creating a temporary vector in getInstructionCase

Record::getValues returns ArrayRef which has a cast operator
to std::vector, as a result a temporary vector is created
if the type of the variable is const std::vector& 
that is suboptimal in this case.

Differential revision: https://reviews.llvm.org/D34969

Test plan: make check-all

llvm-svn: 307063