Subzero: Strength-reduce mul by certain constants.
These all appear to some degree in spec2k.
This is implemented for i8/i16/i32 types. It is done as part of core lowering, so in theory all optimization levels could benefit, but it is explicitly disabled for Om1/O0 to keep things simple there.
While clang appears to strength-reduce udiv/urem by a constant power of 2, for some reason it does not always strength-reduce multiplies (given that they appear in the spec2k bitcode).
For multiplies by 3, 5, or 9, we can make use of the lea instruction. We can do combinations of shift and lea to multiply by other constants, e.g. 100=5*5*4. If too many operations would be required, just give up and use the mul instruction.
BUG= https://code.google.com/p/nativeclient/issues/detail?id=4095
R=jpp@chromium.org, jvoung@chromium.org
Review URL: https://codereview.chromium.org/1146803002
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