vasm Branch fusion

Summary: Simplifications at the vasm level:
  jcc{E,  testb{setcc{cc, flags}}} => jcc{!cc, flags}
  jcc{NE, testb{setcc{cc, flags}}} => jcc{cc,  flags}

Removed HHIR fused-branch instructions.

Reviewed By: @jdelong

Differential Revision: D1758029
Use vasm to fold indexed loads when index is a const.

Summary: Convert load{base[index+disp],d} => load{base[disp],d} if index is
a known const and index+disp doesn't overflow int32.

Reviewed By: @jdelong

Differential Revision: D1758779

Signature: t1:1758779:1419810411:c847b609f59071ea66a0dad978ff02adcd4da7fd