[AIX][TLS] Produce a faster local-exec access sequence with...

[AIX][TLS] Produce a faster local-exec access sequence with -maix-small-local-exec-tls (And optimize when load/store offsets are 0)

This patch utilizes the -maix-small-local-exec-tls option added in
D155544 to produce a faster access sequence for the local-exec TLS
model, where loading from the TOC can be avoided.

The patch either produces an addi/la with a displacement off of r13
(the thread pointer) when the address is calculated, or it produces an
addi/la followed by a load/store when the address is calculated and
used for further accesses.

This patch also optimizes this sequence a bit more where we can remove
the addi/la when the load/store offset is 0. A follow up patch will
be posted to account for when the load/store offset is non-zero, and
currently in these situations we keep the addi/la that precedes the
load/store.

Furthermore, this access sequence is only performed for TLS variables
that are less than ~32KB in size.

Differential Revision: https://reviews.llvm.org/D155600