[X86] Correct spill slot size.

This patch fixes a bug introduced by revision 330778 (originally reviewed at:
https://reviews.llvm.org/D44782), where function isFrameLoadOpcode returned
the wrong number of bytes read for opcodes VMOVSSrm and VMOVSDrm.

This corrects that mistake, and extends the regression test to catch cases where
the dead stores should be removed.

Patch by Jeremy Morse.

Differential Revision: https://reviews.llvm.org/D46256

llvm-svn: 331252