Account for scalarization costs in BasicTTI::getMemoryOpCost for extending vector loads

When a vector type legalizes to a larger vector type, and the target does not
support the associated extending load (or truncating store), then legalization
will scalarize the load (or store) resulting in an associated scalarization
cost.  BasicTTI::getMemoryOpCost needs to account for this.

Between this, and r205487, PowerPC on the P7 with VSX enabled shows:

MultiSource/Benchmarks/PAQ8p/paq8p: 43% speedup
SingleSource/Benchmarks/BenchmarkGame/puzzle: 51% speedup
SingleSource/UnitTests/Vectorizer/gcc-loops 28% speedup

(some of these are new; some of these, such as PAQ8p, just reverse regressions
that VSX support would trigger)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205495 91177308-0d34-0410-b5e6-96231b3b80d8
Revert "[Constant Hoisting] Lazily compute the idom and cache the result."

This code is no longer usefull, because we only compute and use the
IDom once. There is no benefit in caching it anymore.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205498 91177308-0d34-0410-b5e6-96231b3b80d8
Simplify ELFObjectFile<ELFT>::getSymbolAddress.

In particular, we only need to fetch the section if this is a relocatable
object.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205499 91177308-0d34-0410-b5e6-96231b3b80d8