[X86] Refactored stack memory folding tests to explicitly force register spilling

The current 'big vectors' stack folded reload testing pattern is very bulky and makes it difficult to test all instructions as big vectors will tend to use only the ymm instruction implementations.

This patch changes the tests to use a nop call that lists explicit xmm registers as sideeffects, with this we can force a partial register spill of the relevant registers and then check that the reload is correctly folded. The asm generated only adds the forced spill, a nop instruction and a couple of extra labels (a fraction of the current approach).

More exhaustive tests will follow shortly, I've added some extra tests (the xmm versions of some of the existing folding tests) as a starting point.

Differential Revision: http://reviews.llvm.org/D6932

llvm-svn: 226264