Completely eliminate the per-machine-instruction regsUsed set.

This substantially shrinks the size of each machine instruction, which should
make allocation faster and the cache footprint of the machine code lighter.

Here are some timings for code generation of the larger benchmarks we have.
This are timings of code generation phases of the X86 JIT, when compiled in
debug mode:

		Before	After	Diff
164.gzip:
  InstSel	0.0878	0.0722	-21.6%
  RegAlloc	0.2031	0.1757	-15.6%
  TOTAL		0.5585	0.4999	-11.7%
Ptrdist-bc:
  InstSel	0.0878	0.0722	-21.6%
  RegAlloc	0.2070	0.1933	- 7.1%
  TOTAL		0.6972	0.6464	- 7.9%
197.parser:
  InstSel	0.2148	0.2148	- 0.0%
  RegAlloc	0.4941	0.4277	-15.5%
  TOTAL		1.3749	1.2851	- 7.0%
175.vpr:
  InstSel	0.2519	0.2109	-19.4%
  RegAlloc	0.5976	0.5663	- 5.5%
  TOTAL		1.6933	1.6347	- 3.5%
254.gap:
  InstSel	1.1328	0.9921	-14.2%
  RegAlloc	2.6933	2.4804	- 8.6%
  TOTAL		7.7871	7.2499	- 7.4%

llvm-svn: 7622