[SystemZ] Use SRST to implement strlen and strnlen

It would also make sense to use it for memchr; I'm working on that now.
Fix Thumb2 aliasing complementary instructions taking modified immediates

There are many Thumb instructions which take 12-bit immediates encoded in a special
8-byte value + 4-byte rotator form. Not all numbers are represented, and it's legal
to transform an assembly instruction to be able to encode the immediate.

For example: AND and BIC are complementary instructions; one can switch the AND
to a BIC as long as the immediate is complemented.

The intent is to switch one instruction into its complementary one when the immediate
cannot be encoded in the form requested in the original assembly and when the
complementary immediate is encodable.

The patch addresses two issues:
1. definition of t2SOImmNot immediate - it has to check that the orignal value is
not encoded naturally
2. t2AND and t2BIC instruction aliases which should use the Thumb2 SOImm operand
rather than the ARM one.
Add support for Thumb2 literal loads with negative zero offset

Thumb2 literal loads use an offset encoding which allows for
negative zero. This fixes parsing and encoding so that #-0
is correctly processed. The parser represents #-0 as INT32_MIN.
Tweak Replacement comparisons

* Introduce operator< to replace Replacement::Less
* Make operator== and operator< on Replacements non-member functions
* Change order of comparisons in operator< to do string comparisons last
When initializing the PIC global base register on ARM/ELF add pc to fix the address.

This unbreaks PIC with fast isel on ELF targets (PR16717). The output matches
what GCC and SDag do for PIC but may not cover all of the many flavors of PIC
that exist.
Passing false instead of 0; no functional change intended.
Passing false instead of 0; no functional change intended.
Updating function comments; no functional changes intended.
R600: Allocate memoperand in the MachienFunction so it doesn't leak.