[GlobalISel] Handle multiple registers in dbg.value intrinsic

https://reviews.llvm.org/D66077

The value passed into dbg.value may relate to multiple registers,
each of which need a DBG_VALUE.

This fix calls MIRBuilder.buildDirectDbgValue for each register.

Without this, IR passed in from flang-compiler/flang may fail an
assertion in getOrCreateVReg.

Patch by : peterwaller-arm.

llvm-svn: 369403