[RISCV] Set isReMaterializable on ADDI and LUI instructions

The isReMaterlizable flag is somewhat confusing, unlike most other instruction 
flags it is currently interpreted as a hint (mightBeRematerializable would be 
a better name). While LUI is always rematerialisable, for an instruction like 
ADDI it depends on its operands. TargetInstrInfo::isTriviallyReMaterializable 
will call TargetInstrInfo::isReallyTriviallyReMaterializable, which in turn 
calls TargetInstrInfo::isReallyTriviallyReMaterializableGeneric. We rely on 
the logic in the latter to pick out instances of ADDI that really are 
rematerializable.

The isReMaterializable flag does make a difference on a variety of test 
programs. The recently committed remat.ll test case demonstrates how stack 
usage is reduce and a unnecessary lw/sw can be removed. Stack usage in the 
Proc0 function in dhrystone reduces from 192 bytes to 112 bytes.

For the sake of completeness, this patch also implements 
RISCVRegisterInfo::isConstantPhysReg. Although this is called from a number of 
places, it doesn't seem to result in different codegen for any programs I've 
thrown at it. However, it is called in the rematerialisation codepath and it 
seems sensible to implement something correct here.

Differential Revision: https://reviews.llvm.org/D46182

llvm-svn: 332617

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