JIT: use fastmem loads in MMU mode.

Even in games that require MMU mode, loads outside the area specified by
the BAT are rare, so fastmem is a substantial improvement.

All of the interesting changes are in the backpatch handler, to make it
generate DSI exceptions correctly.
JIT: simplify code using MOVTwo.
DiscIO: Clear error status when reading file
Merge pull request #1693 from magumagu/memcheck-fastmem

JIT: use fastmem loads in MMU mode.
Merge pull request #1700 from JosJuice/discio-clear-error

DiscIO: Clear error status when reading file
D3D+OGL: choose blendenable over logicopenable.

No sane game should turn on both logicopenable and blendenable in the same
blend mode, but not every game is sane. Fixes issue 6009.
Merge pull request #1707 from magumagu/blend-and-logic-op

D3D+OGL: choose blendenable over logicopenable.