ARM64: Optimize pair of "str reg, [reg]" to stp

[kunalspathak]

str     x13, [x12]
str     x14, [x12,#8]

can be combined into stp if the stores are happening in subsequent memory.

stp x13, x14, [x12]

I collected no. of such str pairs in framework libraries and found approx. 1000 pairs in 400 methods.

Details:

str_str_x_to_stp.txt

category:cq
theme:optimization
skill-level:intermediate
cost:small
impact:medium