Don't raise inexact when lowering ceil, floor, round, trunc.

The C standard has historically not specified whether or not these functions should raise the inexact flag. Traditionally on Darwin, these functions *did* raise inexact, and the llvm lowerings followed that conventions. n1778 (C bindings for IEEE-754 (2008)) clarifies that these functions should not set inexact. This patch brings the lowerings for arm64 and x86 in line with the newly specified behavior.  This also lets us fold some logic into TD patterns, which is nice.

Differential Revision: http://reviews.llvm.org/D12969

llvm-svn: 248266