[PGO/SamplePGO][NFC] Move the function updateProfWeight from Instruction

to CallInst.

The issue was raised here: https://reviews.llvm.org/D60903#1472783

The function Instruction::updateProfWeight is only used for CallInst in
profile update. From the current interface, it is very easy to think that
the function can also be used for branch instruction. However, Branch
instruction does't need the scaling the function provides for
branch_weights and VP (value profile), in addition, scaling may introduce
inaccuracy for branch probablity.

The patch moves the function updateProfWeight from Instruction class to
CallInst to remove the confusion. The patch also changes the scaling of
branch_weights from a loop to a block because we know that ProfileData
for branch_weights of CallInst will only have two operands at most.

Differential Revision: https://reviews.llvm.org/D60911

llvm-svn: 358900