Z peephole: Remove unnecessary L(L)GFR

A 32 bit load instruction followed by a zero/sign extend instruction can be replaced with an equivalent load and zero/sign extend instruction.

Here are all the replacements done by this peephole:	opcode	`LGFR R1,R1`
`L R1,M1`*	`LGF R1,M1`*	`LLGF R1,M1`*
`LH R1,M1`*	`LGH R1,M1`*	N/A
`LLH R1,M1`*	N/A	`LLGH R1,M1`*
`LB R1,M1`*	`LGB R1,M1`*	N/A
`LLC R1,M1`*	N/A	`LLGC R1,M1`*
`XR R1,R1`	`XGR R1,R1`	`XGR R1,R1`
`IILF R1,I1`	`LGFI R1,I1`	`LLILF R1,I1`
`LHI R1,I1`	`LGHI R1,I1`	`LLILF R1,I2`**
`LR R1,R2`	`LGFR R1,R2`	`LLGFR R1,R2`
`LGR R1,R2`	`LGFR R1,R2`	`LLGFR R1,R2`

*M1 is an arbitrary memory reference **I2 is I1 sign extended from 16 to 32 bits

eclipse / omr