[PowerPC] Eliminate sign- and zero-extensions if already sign- or zero-extended

This patch enables redundant sign- and zero-extension elimination in PowerPC MI Peephole pass.
If the input value of a sign- or zero-extension is known to be already sign- or zero-extended, the operation is redundant and can be eliminated.
One common case is sign-extensions for a method parameter or for a method return value; they must be sign- or zero-extended as defined in PPC ELF ABI. 
For example of the following simple code, two extsw instructions are generated before the invocation of int_func and before the return. With this patch, both extsw are eliminated.

void int_func(int);
void ii_test(int a) {
    if (a & 1) return int_func(a);
}

Such redundant sign- or zero-extensions are quite common in many programs; e.g. I observed about 60,000 occurrences of the elimination while compiling the LLVM+CLANG.

Differential Revision: https://reviews.llvm.org/D31319

llvm-svn: 315888