ARM: Fix a bug which was causing convergence failure in constant-island pass.

The bug is in ARMConstantIslands::createNewWater where the upper bound of the
new water split point is computed:

// This could point off the end of the block if we've already got constant
// pool entries following this block; only the last one is in the water list.
// Back past any possible branches (allow for a conditional and a maximally
// long unconditional).
if (BaseInsertOffset + 8 >= UserBBI.postOffset()) {
  BaseInsertOffset = UserBBI.postOffset() - UPad - 8;
  DEBUG(dbgs() << format("Move inside block: %#x\n", BaseInsertOffset));
}

The split point is supposed to be somewhere between the machine instruction that
loads from the constant pool entry and the end of the basic block, before branch
instructions. The code above is fine if the basic block is large enough and
there are a sufficient number of instructions following the machine instruction.
However, if the machine instruction is near the end of the basic block,
BaseInsertOffset can point to the machine instruction or another instruction
that precedes it, and this can lead to convergence failure.

This commit fixes this bug by ensuring BaseInsertOffset is larger than the
offset of the instruction following the constant-loading instruction.

rdar://problem/18581150


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220015 91177308-0d34-0410-b5e6-96231b3b80d8
Add a test that would have found the bug in r219829.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220016 91177308-0d34-0410-b5e6-96231b3b80d8
[PPC] Adjust some PowerPC tests to account for presence/absence of VSX

Patch by Bill Seurer; committed on his behalf.

These test cases generate slightly different code sequences when VSX
is activated and thus fail. The update turns off VSX explicitly for
the existing checks and then adds a second set of checks for most of
them that test the VSX instruction output.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220019 91177308-0d34-0410-b5e6-96231b3b80d8
Add back commits r219835 and a fixed version of r219829.

The only difference from r219829 is using

getOrCreateSectionSymbol(*ELFSec)

instead of

GetOrCreateSymbol(ELFSec->getSectionName())

in ELFObjectWriter which causes us to use the correct section symbol even if
we have multiple sections with the same name.

Original messages:

r219829:
Correctly handle references to section symbols.

When processing assembly like

.long .text

we were creating a new undefined symbol .text. GAS on the other hand would
handle that as a reference to the .text section.

This patch implements that by creating the section symbols earlier so that
they are visible during asm parsing.

The patch also updates llvm-readobj to print the symbol number in the relocation
dump so that the test can differentiate between two sections with the same name.

r219835:
Allow forward references to section symbols.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220021 91177308-0d34-0410-b5e6-96231b3b80d8